diff --git a/beacon_chain/fork_choice_rule/fork_choice_rule.nim b/beacon_chain/fork_choice_rule/fork_choice_rule.nim
index 13f752af0..928e80754 100644
--- a/beacon_chain/fork_choice_rule/fork_choice_rule.nim
+++ b/beacon_chain/fork_choice_rule/fork_choice_rule.nim
@@ -13,6 +13,7 @@
 import
   # Stdlib
   tables, deques, strutils, endians, strformat,
+  times,
   # Nimble packages
   nimcrypto,
   # Local imports
@@ -27,7 +28,7 @@ method on_receive(self: Node, obj: BlockOrSig, reprocess = false) {.base.} =
 ###########################################################
 
 proc broadcast(self: Node, x: Block) =
-  if self.sleepy and self.timestamp != 0:
+  if self.sleepy and self.timestamp != Duration():
     return
   self.network.broadcast(self, x)
   self.on_receive(x)
@@ -191,7 +192,8 @@ method on_receive(self: Node, sig: Sig, reprocess = false) =
       # considered finalized
 
       var finalize = true
-      for slot2 in countdown(slot-1, max(slot - EPOCH_LENGTH * 1, 0) - 1):
+      for slot2 in countdown(slot-1, max(slot - EPOCH_LENGTH * 1, 0)):
+        # Note the max(...)-1 in spec is unneeded, Nim ranges are inclusive
         if slot2 < self.blocks[c2].slot:
           c2 = self.blocks[c2].parent_hash
 
@@ -223,3 +225,19 @@ method on_receive(self: Node, sig: Sig, reprocess = false) =
 
   # Rebroadcast
   self.network.broadcast(self, sig)
+
+func get_sig_targets(self: Node, start_slot: int32): seq[MDigest[256]] =
+  # Get the portion of the main chain that is within the last EPOCH_LENGTH
+  # slots, once again duplicating the parent in cases where the parent and
+  # child's slots are not consecutive
+  result = @[]
+  var i = self.main_chain.high
+  for slot in countdown(start_slot-1, max(start_slot - EPOCH_LENGTH, 0)):
+    # Note the max(...)-1 in spec is unneeded, Nim ranges are inclusive
+    if slot < self.blocks[self.main_chain[i]].slot:
+      dec i
+    result.add self.main_chain[i]
+  for i, x in result:
+    doAssert self.blocks[x].slot <= start_slot - 1 - i
+  doAssert result.len == min(EPOCH_LENGTH, start_slot)
+
diff --git a/beacon_chain/fork_choice_rule/fork_choice_types.nim b/beacon_chain/fork_choice_rule/fork_choice_types.nim
index b96eb3372..c82b08173 100644
--- a/beacon_chain/fork_choice_rule/fork_choice_types.nim
+++ b/beacon_chain/fork_choice_rule/fork_choice_types.nim
@@ -11,13 +11,13 @@
 # Note that implementation is not updated to the latest v2.1 yet
 
 import
-  tables, deques, strutils, hashes,  # Stdlib
-  nimcrypto                          # Nimble packages
+  tables, deques, strutils, hashes, times, # Stdlib
+  nimcrypto                                # Nimble packages
 
 const
-  NOTARIES* = 100    # Committee size in Casper v2.1
-  SLOT_SIZE* = 6     # Slot duration in Casper v2.1
-  EPOCH_LENGTH* = 25 # Cycle length inCasper v2.
+  NOTARIES* = 100                        # Committee size in Casper v2.1
+  SLOT_SIZE* = initDuration(seconds = 6) # Slot duration in Casper v2.1
+  EPOCH_LENGTH* = 25                     # Cycle length in Casper v2.
 
 # TODO, clear up if reference semantics are needed
 # for the tables. I.e. what's their maximum size.
@@ -43,7 +43,7 @@ type
     slot*: int64
 ##########################################
 
-func min_timestamp*(self: Block): int64 =
+func min_timestamp*(self: Block): Duration =
   SLOT_SIZE * self.slot
 
 let Genesis* = Block()
@@ -71,14 +71,20 @@ method hash*(x: SigHash): Hash =
   ## Allow usage of Sighash in tables
   x.raw.hash
 
+func hash*(x: Duration): Hash =
+  ## Allow usage of Duration in tables
+  # Due to rpivate fields, we use pointer + length as a hack:
+  # https://github.com/nim-lang/Nim/issues/8857
+  result = hashData(x.unsafeAddr, x.sizeof)
+
 #########################################
 
 type
   NetworkSimulator* = ref object
     agents*: seq[int]
-    latency_distribution_sample*: proc (): int
-    time*: int64
-    objqueue*: TableRef[int64, seq[(Node, BlockOrSig)]]
+    latency_distribution_sample*: proc (): Duration
+    time*: Duration
+    objqueue*: TableRef[Duration, seq[(Node, BlockOrSig)]]
     peers*: TableRef[int, seq[Node]]
     reliability*: float
 
@@ -101,7 +107,7 @@ type
     scores_at_height*: TableRef[array[36, byte], int] # Should be slot not height in v2.1
     justified*: TableRef[MDigest[256], bool]
     finalized*: TableRef[MDigest[256], bool]
-    timestamp*: int64
+    timestamp*: Duration
     id*: int
     network*: NetworkSimulator
     used_parents*: TableRef[MDigest[256], Node]
diff --git a/beacon_chain/fork_choice_rule/networksim.nim b/beacon_chain/fork_choice_rule/networksim.nim
index 58124022d..4ab86db37 100644
--- a/beacon_chain/fork_choice_rule/networksim.nim
+++ b/beacon_chain/fork_choice_rule/networksim.nim
@@ -10,7 +10,7 @@
 # Part of Casper+Sharding chain v2.1: https://notes.ethereum.org/SCIg8AH5SA-O4C1G1LYZHQ#
 
 import
-  tables,
+  tables, times,
   ./fork_choice_types
 
 func broadcast*(self: NetworkSimulator, sender: Node, obj: BlockOrSig) =