diff --git a/contracts/Proofs.sol b/contracts/Proofs.sol
index a4fdde2..1315c05 100644
--- a/contracts/Proofs.sol
+++ b/contracts/Proofs.sol
@@ -6,6 +6,7 @@ contract Proofs {
   mapping(bytes32=>bool) private ids;
   mapping(bytes32=>uint) private periods;
   mapping(bytes32=>uint) private timeouts;
+  mapping(bytes32=>uint) private starts;
   mapping(bytes32=>uint) private ends;
   mapping(bytes32=>uint) private markers;
   mapping(bytes32=>uint) private missed;
@@ -46,6 +47,7 @@ contract Proofs {
     ids[id] = true;
     periods[id] = period;
     timeouts[id] = timeout;
+    starts[id] = block.number;
     ends[id] = block.number + duration + 2 * timeout;
     markers[id] = uint(blockhash(block.number - 1)) % period;
   }
@@ -61,7 +63,7 @@ contract Proofs {
     internal view
     returns (bool)
   {
-    if (block.number >= ends[id]) {
+    if (blocknumber < starts[id] || blocknumber >= ends[id]) {
       return false;
     }
     bytes32 hash = blockhash(blocknumber - 1);
diff --git a/test/Proofs.test.js b/test/Proofs.test.js
index 984d0a2..ca7a5e1 100644
--- a/test/Proofs.test.js
+++ b/test/Proofs.test.js
@@ -64,6 +64,17 @@ describe("Proofs", function () {
     expect(average).to.be.closeTo(period, period / 2)
   })
 
+  it("requires no proof before start time", async function () {
+    for (let i=0; i<4*period; i++) {
+      mineBlock()
+    }
+    await proofs.expectProofs(id, period, timeout, duration)
+    let start = await minedBlockNumber()
+    for (let i=1; i<4*period; i++) {
+      expect(await proofs.isProofRequired(id, start-i)).to.be.false
+    }
+  })
+
   describe("when proofs are required", async function () {
 
     beforeEach(async function () {