diff --git a/contracts/Proofs.sol b/contracts/Proofs.sol
index 5c7ff95..df8120e 100644
--- a/contracts/Proofs.sol
+++ b/contracts/Proofs.sol
@@ -13,6 +13,7 @@ contract Proofs {
   mapping(bytes32 => bool) private ids;
   mapping(bytes32 => uint256) private starts;
   mapping(bytes32 => uint256) private ends;
+  mapping(bytes32 => uint256) private probabilities;
   mapping(bytes32 => uint256) private markers;
   mapping(bytes32 => uint256) private missed;
   mapping(bytes32 => mapping(uint256 => bool)) private received;
@@ -34,55 +35,95 @@ contract Proofs {
     return missed[id];
   }
 
-  function _expectProofs(bytes32 id, uint256 duration) internal {
+  function periodOf(uint256 timestamp) private view returns (uint256) {
+    return timestamp / period;
+  }
+
+  function currentPeriod() private view returns (uint256) {
+    return periodOf(block.timestamp);
+  }
+
+  function _expectProofs(
+    bytes32 id,
+    uint256 probability,
+    uint256 duration
+  ) internal {
     require(!ids[id], "Proof id already in use");
     ids[id] = true;
-    starts[id] = block.number;
-    ends[id] = block.number + duration + 2 * timeout;
+    starts[id] = block.timestamp;
+    ends[id] = block.timestamp + duration;
+    probabilities[id] = probability;
     markers[id] = uint256(blockhash(block.number - 1)) % period;
   }
 
-  // Check whether a proof is required at the time of the block with the
-  // specified block number. A proof has to be submitted within the proof
-  // timeout for it to be valid. Whether a proof is required is determined
-  // randomly, but on average it is once every proof period.
-  function _isProofRequired(bytes32 id, uint256 blocknumber)
+  function _getChallenges(bytes32 id, uint256 proofperiod)
+    internal
+    view
+    returns (Challenge memory challenge1, Challenge memory challenge2)
+  {
+    if (
+      proofperiod <= periodOf(starts[id]) || proofperiod >= periodOf(ends[id])
+    ) {
+      bytes32 nullChallenge;
+      return (Challenge(false, nullChallenge), Challenge(false, nullChallenge));
+    }
+
+    uint256 blocknumber = block.number % 256;
+    uint256 periodnumber = proofperiod % 256;
+    uint256 idoffset = uint256(id) % 256;
+
+    uint256 pointer1 = (blocknumber + periodnumber + idoffset) % 256;
+    uint256 pointer2 = (blocknumber + periodnumber + idoffset + 128) % 256;
+
+    bytes32 blockhash1 = blockhash(block.number - 1 - pointer1);
+    bytes32 blockhash2 = blockhash(block.number - 1 - pointer2);
+
+    assert(uint256(blockhash1) != 0);
+    assert(uint256(blockhash2) != 0);
+
+    challenge1.challenge = keccak256(abi.encode(blockhash1));
+    challenge2.challenge = keccak256(abi.encode(blockhash2));
+
+    challenge1.isProofRequired =
+      uint256(challenge1.challenge) % probabilities[id] == 0;
+    challenge2.isProofRequired =
+      uint256(challenge2.challenge) % probabilities[id] == 0;
+  }
+
+  function _isProofRequired(bytes32 id, uint256 proofPeriod)
     internal
     view
     returns (bool)
   {
-    if (blocknumber < starts[id] || blocknumber >= ends[id]) {
-      return false;
-    }
-    bytes32 hash = blockhash(blocknumber - 1);
-    return hash != 0 && uint256(hash) % period == markers[id];
+    Challenge memory challenge1;
+    Challenge memory challenge2;
+    (challenge1, challenge2) = _getChallenges(id, proofPeriod);
+    return challenge1.isProofRequired && challenge2.isProofRequired;
   }
 
-  function _isProofTimedOut(uint256 blocknumber) internal view returns (bool) {
-    return block.number >= blocknumber + timeout;
+  function _isProofRequired(bytes32 id) internal view returns (bool) {
+    return _isProofRequired(id, currentPeriod());
   }
 
-  function _submitProof(
-    bytes32 id,
-    uint256 blocknumber,
-    bool proof
-  ) internal {
+  function _submitProof(bytes32 id, bool proof) internal {
     require(proof, "Invalid proof"); // TODO: replace bool by actual proof
-    require(
-      _isProofRequired(id, blocknumber),
-      "No proof required for this block"
-    );
-    require(!_isProofTimedOut(blocknumber), "Proof not allowed after timeout");
-    require(!received[id][blocknumber], "Proof already submitted");
-    received[id][blocknumber] = true;
+    require(!received[id][currentPeriod()], "Proof already submitted");
+    received[id][currentPeriod()] = true;
   }
 
-  function _markProofAsMissing(bytes32 id, uint256 blocknumber) internal {
-    require(_isProofTimedOut(blocknumber), "Proof has not timed out yet");
-    require(!received[id][blocknumber], "Proof was submitted, not missing");
-    require(_isProofRequired(id, blocknumber), "Proof was not required");
-    require(!missing[id][blocknumber], "Proof already marked as missing");
-    missing[id][blocknumber] = true;
+  function _markProofAsMissing(bytes32 id, uint256 missedPeriod) internal {
+    uint256 periodEnd = (missedPeriod + 1) * period;
+    require(periodEnd < block.timestamp, "Period has not ended yet");
+    require(block.timestamp < periodEnd + timeout, "Validation timed out");
+    require(!received[id][missedPeriod], "Proof was submitted, not missing");
+    require(_isProofRequired(id, missedPeriod), "Proof was not required");
+    require(!missing[id][missedPeriod], "Proof already marked as missing");
+    missing[id][missedPeriod] = true;
     missed[id] += 1;
   }
+
+  struct Challenge {
+    bool isProofRequired;
+    bytes32 challenge;
+  }
 }
diff --git a/contracts/Storage.sol b/contracts/Storage.sol
index 3ac8ec1..de2a0fd 100644
--- a/contracts/Storage.sol
+++ b/contracts/Storage.sol
@@ -29,11 +29,11 @@ contract Storage is Collateral, Marketplace, Proofs {
     Offer storage offer = _offer(id);
     require(msg.sender == offer.host, "Only host can call this function");
     Request storage request = _request(offer.requestId);
-    _expectProofs(id, request.duration);
+    _expectProofs(id, request.proofProbability, request.duration);
   }
 
   function finishContract(bytes32 id) public {
-    require(block.number > proofEnd(id), "Contract has not ended yet");
+    require(block.timestamp > proofEnd(id), "Contract has not ended yet");
     require(!finished[id], "Contract already finished");
     finished[id] = true;
     Offer storage offer = _offer(id);
@@ -56,28 +56,16 @@ contract Storage is Collateral, Marketplace, Proofs {
     return _missed(contractId);
   }
 
-  function isProofRequired(bytes32 contractId, uint256 blocknumber)
-    public
-    view
-    returns (bool)
-  {
-    return _isProofRequired(contractId, blocknumber);
+  function isProofRequired(bytes32 contractId) public view returns (bool) {
+    return _isProofRequired(contractId);
   }
 
-  function isProofTimedOut(uint256 blocknumber) public view returns (bool) {
-    return _isProofTimedOut(blocknumber);
+  function submitProof(bytes32 contractId, bool proof) public {
+    _submitProof(contractId, proof);
   }
 
-  function submitProof(
-    bytes32 contractId,
-    uint256 blocknumber,
-    bool proof
-  ) public {
-    _submitProof(contractId, blocknumber, proof);
-  }
-
-  function markProofAsMissing(bytes32 contractId, uint256 blocknumber) public {
-    _markProofAsMissing(contractId, blocknumber);
+  function markProofAsMissing(bytes32 contractId, uint256 period) public {
+    _markProofAsMissing(contractId, period);
     if (_missed(contractId) % slashMisses == 0) {
       Offer storage offer = _offer(contractId);
       _slash(offer.host, slashPercentage);
diff --git a/contracts/TestProofs.sol b/contracts/TestProofs.sol
index 2641131..650004f 100644
--- a/contracts/TestProofs.sol
+++ b/contracts/TestProofs.sol
@@ -28,27 +28,23 @@ contract TestProofs is Proofs {
     return _missed(id);
   }
 
-  function expectProofs(bytes32 id, uint256 _duration) public {
-    _expectProofs(id, _duration);
-  }
-
-  function isProofRequired(bytes32 id, uint256 blocknumber)
-    public
-    view
-    returns (bool)
-  {
-    return _isProofRequired(id, blocknumber);
-  }
-
-  function submitProof(
+  function expectProofs(
     bytes32 id,
-    uint256 blocknumber,
-    bool proof
+    uint256 _probability,
+    uint256 _duration
   ) public {
-    _submitProof(id, blocknumber, proof);
+    _expectProofs(id, _probability, _duration);
   }
 
-  function markProofAsMissing(bytes32 id, uint256 blocknumber) public {
-    _markProofAsMissing(id, blocknumber);
+  function isProofRequired(bytes32 id) public view returns (bool) {
+    return _isProofRequired(id);
+  }
+
+  function submitProof(bytes32 id, bool proof) public {
+    _submitProof(id, proof);
+  }
+
+  function markProofAsMissing(bytes32 id, uint256 _period) public {
+    _markProofAsMissing(id, _period);
   }
 }
diff --git a/test/Proofs.test.js b/test/Proofs.test.js
index 5124894..24a0fb8 100644
--- a/test/Proofs.test.js
+++ b/test/Proofs.test.js
@@ -1,190 +1,202 @@
 const { expect } = require("chai")
 const { ethers } = require("hardhat")
 const { mineBlock, minedBlockNumber } = require("./evm")
+const {
+  snapshot,
+  revert,
+  currentTime,
+  advanceTime,
+  advanceTimeTo,
+} = require("./evm")
 
 describe("Proofs", function () {
   const id = ethers.utils.randomBytes(32)
   const period = 10
   const timeout = 5
-  const duration = 50
+  const duration = 1000
+  const probability = 2 // require a proof roughly once every 2² periods
 
   let proofs
 
+  async function ensureEnoughBlockHistory() {
+    while ((await minedBlockNumber()) < 256) {
+      await mineBlock()
+    }
+  }
+
   beforeEach(async function () {
+    await snapshot()
+    await ensureEnoughBlockHistory()
     const Proofs = await ethers.getContractFactory("TestProofs")
     proofs = await Proofs.deploy(period, timeout)
   })
 
-  it("calculates an end time based on duration and timeout", async function () {
-    await proofs.expectProofs(id, duration)
-    let start = await minedBlockNumber()
-    let end = start + duration + 2 * timeout
-    expect(await proofs.end(id)).to.equal(end)
+  afterEach(async function () {
+    await revert()
+  })
+
+  it("calculates an end time based on duration", async function () {
+    await proofs.expectProofs(id, probability, duration)
+    let end = (await currentTime()) + duration
+    expect((await proofs.end(id)).toNumber()).to.be.closeTo(end, 1)
   })
 
   it("does not allow ids to be reused", async function () {
-    await proofs.expectProofs(id, duration)
-    await expect(proofs.expectProofs(id, duration)).to.be.revertedWith(
-      "Proof id already in use"
-    )
+    await proofs.expectProofs(id, probability, duration)
+    await expect(
+      proofs.expectProofs(id, probability, duration)
+    ).to.be.revertedWith("Proof id already in use")
   })
 
-  it("requires on average a proof every period", async function () {
-    let blocks = 600
+  it("requires proofs with an agreed upon probability", async function () {
+    const duration = 100_000
+    await proofs.expectProofs(id, probability, duration)
     let amount = 0
-    await proofs.expectProofs(id, blocks)
-    for (let i = 0; i < blocks; i++) {
-      await mineBlock()
-      if (await proofs.isProofRequired(id, await minedBlockNumber())) {
+    for (let i = 0; i < 100; i++) {
+      if (await proofs.isProofRequired(id)) {
         amount += 1
       }
+      await advanceTime(period)
     }
-    let average = blocks / amount
-    expect(average).to.be.closeTo(period, period / 2)
+    let expected = 100 / probability ** 2
+    expect(amount).to.be.closeTo(expected, expected / 2)
   })
 
-  it("requires no proof before start time", async function () {
-    for (let i = 0; i < 4 * period; i++) {
-      mineBlock()
+  it("requires no proofs in the start period", async function () {
+    const startPeriod = Math.floor((await currentTime()) / period)
+    const probability = 1
+    await proofs.expectProofs(id, probability, duration)
+    while (Math.floor((await currentTime()) / period) == startPeriod) {
+      expect(await proofs.isProofRequired(id)).to.be.false
+      await advanceTime(1)
     }
-    await proofs.expectProofs(id, duration)
-    let start = await minedBlockNumber()
-    for (let i = 1; i < 4 * period; i++) {
-      expect(await proofs.isProofRequired(id, start - i)).to.be.false
+  })
+
+  it("requires no proofs in the end period", async function () {
+    const probability = 1
+    await proofs.expectProofs(id, probability, duration)
+    await advanceTime(duration)
+    expect(await proofs.isProofRequired(id)).to.be.false
+  })
+
+  it("requires no proofs after the end time", async function () {
+    const probability = 1
+    await proofs.expectProofs(id, probability, duration)
+    await advanceTime(duration + timeout)
+    expect(await proofs.isProofRequired(id)).to.be.false
+  })
+
+  it("requires proofs for different ids at different times", async function () {
+    let id1 = ethers.utils.randomBytes(32)
+    let id2 = ethers.utils.randomBytes(32)
+    let id3 = ethers.utils.randomBytes(32)
+    for (let id of [id1, id2, id3]) {
+      await proofs.expectProofs(id, probability, duration)
+    }
+    let req1, req2, req3
+    while (req1 === req2 && req2 === req3) {
+      req1 = await proofs.isProofRequired(id1)
+      req2 = await proofs.isProofRequired(id2)
+      req3 = await proofs.isProofRequired(id3)
+      await advanceTime(period)
     }
   })
 
   describe("when proofs are required", async function () {
     beforeEach(async function () {
-      await proofs.expectProofs(id, duration)
+      await proofs.expectProofs(id, probability, duration)
     })
 
-    async function mineUntilProofIsRequired(id) {
-      while (!(await proofs.isProofRequired(id, await minedBlockNumber()))) {
-        mineBlock()
+    async function waitUntilProofIsRequired(id) {
+      while (!(await proofs.isProofRequired(id))) {
+        await advanceTime(period)
       }
     }
 
-    async function mineUntilProofTimeout() {
-      for (let i = 0; i < timeout; i++) {
-        mineBlock()
-      }
+    function periodOf(timestamp) {
+      return Math.floor(timestamp / period)
     }
 
-    async function mineUntilEnd() {
-      const end = await proofs.end(id)
-      while ((await minedBlockNumber()) < end) {
-        mineBlock()
-      }
+    function periodStart(p) {
+      return period * p
     }
 
-    it("requires no proof for blocks that are unavailable", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      for (let i = 0; i < 256; i++) {
-        // only last 256 blocks available in solidity
-        mineBlock()
-      }
-      expect(await proofs.isProofRequired(id, blocknumber)).to.be.false
-    })
-
-    it("requires no proof after end time", async function () {
-      await mineUntilEnd()
-      for (let i = 0; i < 4 * period; i++) {
-        const blocknumber = await minedBlockNumber()
-        expect(await proofs.isProofRequired(id, blocknumber)).to.be.false
-        mineBlock()
-      }
-    })
+    function periodEnd(p) {
+      return periodStart(p + 1)
+    }
 
     it("submits a correct proof", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await proofs.submitProof(id, blocknumber, true)
+      await proofs.submitProof(id, true)
     })
 
     it("fails proof submission when proof is incorrect", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await expect(
-        proofs.submitProof(id, blocknumber, false)
-      ).to.be.revertedWith("Invalid proof")
-    })
-
-    it("fails proof submission when proof was not required", async function () {
-      while (await proofs.isProofRequired(id, await minedBlockNumber())) {
-        await mineBlock()
-      }
-      let blocknumber = await minedBlockNumber()
-      await expect(
-        proofs.submitProof(id, blocknumber, true)
-      ).to.be.revertedWith("No proof required")
-    })
-
-    it("fails proof submission when proof is too late", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await mineUntilProofTimeout()
-      await expect(
-        proofs.submitProof(id, blocknumber, true)
-      ).to.be.revertedWith("Proof not allowed after timeout")
+      await expect(proofs.submitProof(id, false)).to.be.revertedWith(
+        "Invalid proof"
+      )
     })
 
     it("fails proof submission when already submitted", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await proofs.submitProof(id, blocknumber, true)
-      await expect(
-        proofs.submitProof(id, blocknumber, true)
-      ).to.be.revertedWith("Proof already submitted")
+      await advanceTimeTo(periodEnd(periodOf(await currentTime())))
+      await proofs.submitProof(id, true)
+      await expect(proofs.submitProof(id, true)).to.be.revertedWith(
+        "Proof already submitted"
+      )
     })
 
     it("marks a proof as missing", async function () {
       expect(await proofs.missed(id)).to.equal(0)
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await mineUntilProofTimeout()
-      await proofs.markProofAsMissing(id, blocknumber)
+      await waitUntilProofIsRequired(id)
+      let missedPeriod = periodOf(await currentTime())
+      await advanceTimeTo(periodEnd(missedPeriod))
+      await proofs.markProofAsMissing(id, missedPeriod)
       expect(await proofs.missed(id)).to.equal(1)
     })
 
-    it("does not mark a proof as missing before timeout", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await mineBlock()
+    it("does not mark a proof as missing before period end", async function () {
+      await waitUntilProofIsRequired(id)
+      let currentPeriod = periodOf(await currentTime())
       await expect(
-        proofs.markProofAsMissing(id, blocknumber)
-      ).to.be.revertedWith("Proof has not timed out yet")
+        proofs.markProofAsMissing(id, currentPeriod)
+      ).to.be.revertedWith("Period has not ended yet")
+    })
+
+    it("does not mark a proof as missing after timeout", async function () {
+      await waitUntilProofIsRequired(id)
+      let currentPeriod = periodOf(await currentTime())
+      await advanceTimeTo(periodEnd(currentPeriod) + timeout)
+      await expect(
+        proofs.markProofAsMissing(id, currentPeriod)
+      ).to.be.revertedWith("Validation timed out")
     })
 
     it("does not mark a submitted proof as missing", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await proofs.submitProof(id, blocknumber, true)
-      await mineUntilProofTimeout()
+      await waitUntilProofIsRequired(id)
+      let submittedPeriod = periodOf(await currentTime())
+      await proofs.submitProof(id, true)
+      await advanceTimeTo(periodEnd(submittedPeriod))
       await expect(
-        proofs.markProofAsMissing(id, blocknumber)
+        proofs.markProofAsMissing(id, submittedPeriod)
       ).to.be.revertedWith("Proof was submitted, not missing")
     })
 
     it("does not mark proof as missing when not required", async function () {
-      while (await proofs.isProofRequired(id, await minedBlockNumber())) {
-        mineBlock()
+      while (await proofs.isProofRequired(id)) {
+        await advanceTime(period)
       }
-      let blocknumber = await minedBlockNumber()
-      await mineUntilProofTimeout()
+      let currentPeriod = periodOf(await currentTime())
+      await advanceTimeTo(periodEnd(currentPeriod))
       await expect(
-        proofs.markProofAsMissing(id, blocknumber)
+        proofs.markProofAsMissing(id, currentPeriod)
       ).to.be.revertedWith("Proof was not required")
     })
 
     it("does not mark proof as missing twice", async function () {
-      await mineUntilProofIsRequired(id)
-      let blocknumber = await minedBlockNumber()
-      await mineUntilProofTimeout()
-      await proofs.markProofAsMissing(id, blocknumber)
+      await waitUntilProofIsRequired(id)
+      let missedPeriod = periodOf(await currentTime())
+      await advanceTimeTo(periodEnd(missedPeriod))
+      await proofs.markProofAsMissing(id, missedPeriod)
       await expect(
-        proofs.markProofAsMissing(id, blocknumber)
+        proofs.markProofAsMissing(id, missedPeriod)
       ).to.be.revertedWith("Proof already marked as missing")
     })
   })
diff --git a/test/Storage.test.js b/test/Storage.test.js
index 7e21e63..b4bbd08 100644
--- a/test/Storage.test.js
+++ b/test/Storage.test.js
@@ -1,7 +1,13 @@
 const { expect } = require("chai")
 const { ethers, deployments } = require("hardhat")
 const { exampleRequest, exampleOffer } = require("./examples")
-const { mineBlock, minedBlockNumber } = require("./evm")
+const {
+  mineBlock,
+  minedBlockNumber,
+  advanceTime,
+  advanceTimeTo,
+  currentTime,
+} = require("./evm")
 const { requestId, offerId } = require("./ids")
 
 describe("Storage", function () {
@@ -17,6 +23,12 @@ describe("Storage", function () {
     storage = storage.connect(account)
   }
 
+  async function ensureEnoughBlockHistory() {
+    while ((await minedBlockNumber()) < 256) {
+      await mineBlock()
+    }
+  }
+
   beforeEach(async function () {
     ;[client, host] = await ethers.getSigners()
 
@@ -48,6 +60,8 @@ describe("Storage", function () {
     switchAccount(client)
     await storage.selectOffer(offerId(offer))
     id = offerId(offer)
+
+    await ensureEnoughBlockHistory()
   })
 
   describe("starting the contract", function () {
@@ -77,11 +91,9 @@ describe("Storage", function () {
       await storage.startContract(id)
     })
 
-    async function mineUntilEnd() {
-      const end = await storage.proofEnd(id)
-      while ((await minedBlockNumber()) < end) {
-        await mineBlock()
-      }
+    async function waitUntilEnd() {
+      const end = (await storage.proofEnd(id)).toNumber()
+      await advanceTimeTo(end)
     }
 
     // it("unlocks the host collateral", async function () {
@@ -91,7 +103,7 @@ describe("Storage", function () {
     // })
 
     it("pays the host", async function () {
-      await mineUntilEnd()
+      await waitUntilEnd()
       const startBalance = await token.balanceOf(host.address)
       await storage.finishContract(id)
       const endBalance = await token.balanceOf(host.address)
@@ -105,7 +117,7 @@ describe("Storage", function () {
     })
 
     it("can only be done once", async function () {
-      await mineUntilEnd()
+      await waitUntilEnd()
       await storage.finishContract(id)
       await expect(storage.finishContract(id)).to.be.revertedWith(
         "Contract already finished"
@@ -114,20 +126,34 @@ describe("Storage", function () {
   })
 
   describe("slashing when missing proofs", function () {
-    beforeEach(function () {
+    let period
+
+    beforeEach(async function () {
       switchAccount(host)
+      period = (await storage.proofPeriod()).toNumber()
     })
 
+    function periodOf(timestamp) {
+      return Math.floor(timestamp / period)
+    }
+
+    function periodStart(p) {
+      return period * p
+    }
+
+    function periodEnd(p) {
+      return periodStart(p + 1)
+    }
+
     async function ensureProofIsMissing() {
-      while (!(await storage.isProofRequired(id, await minedBlockNumber()))) {
-        mineBlock()
+      let currentPeriod = periodOf(await currentTime())
+      await advanceTimeTo(periodEnd(currentPeriod))
+      while (!(await storage.isProofRequired(id))) {
+        await advanceTime(period)
       }
-      const blocknumber = await minedBlockNumber()
-      const timeout = await storage.proofTimeout()
-      for (let i = 0; i < timeout; i++) {
-        mineBlock()
-      }
-      await storage.markProofAsMissing(id, blocknumber)
+      let missedPeriod = periodOf(await currentTime())
+      await advanceTime(period)
+      await storage.markProofAsMissing(id, missedPeriod)
     }
 
     it("reduces collateral when too many proofs are missing", async function () {
diff --git a/test/evm.js b/test/evm.js
index c9588d4..e92090a 100644
--- a/test/evm.js
+++ b/test/evm.js
@@ -32,6 +32,13 @@ async function advanceTime(seconds) {
   await mineBlock()
 }
 
+async function advanceTimeTo(timestamp) {
+  if ((await currentTime()) !== timestamp) {
+    ethers.provider.send("evm_setNextBlockTimestamp", [timestamp])
+    await mineBlock()
+  }
+}
+
 module.exports = {
   snapshot,
   revert,
@@ -39,4 +46,5 @@ module.exports = {
   minedBlockNumber,
   currentTime,
   advanceTime,
+  advanceTimeTo,
 }
diff --git a/test/examples.js b/test/examples.js
index e0e3710..a1e3c06 100644
--- a/test/examples.js
+++ b/test/examples.js
@@ -4,10 +4,10 @@ const { sha256, hexlify, randomBytes } = ethers.utils
 
 const exampleRequest = () => ({
   client: hexlify(randomBytes(20)),
-  duration: 150, // 150 blocks ≈ half an hour
+  duration: hours(10),
   size: 1 * 1024 * 1024 * 1024, // 1 Gigabyte
   contentHash: sha256("0xdeadbeef"),
-  proofProbability: 5, // require a proof roughly once every 5^2 periods
+  proofProbability: 2, // require a proof roughly once every 2² periods
   maxPrice: 84,
   expiry: now() + hours(1),
   nonce: hexlify(randomBytes(32)),