diff --git a/beacon-chain/core/blocks/validity_conditions.go b/beacon-chain/core/blocks/validity_conditions.go
index 69b87f9c6..9df80fd97 100644
--- a/beacon-chain/core/blocks/validity_conditions.go
+++ b/beacon-chain/core/blocks/validity_conditions.go
@@ -56,8 +56,10 @@ func IsValidBlock(
 	}
 
 	// Pre-Processing Condition 4:
-	// The node's local time is greater than or equal to
-	// state.genesis_time + (block.slot-GENESIS_SLOT)* SLOT_DURATION.
+	// The node's local Unix time is greater than or equal to
+	// state.genesis_time + (block.slot-GENESIS_SLOT) * SLOT_DURATION.
+	// (Note that leap seconds mean that slots will occasionally last SLOT_DURATION + 1 or
+	// SLOT_DURATION - 1 seconds, possibly several times a year.)
 	if !IsSlotValid(block.Slot, genesisTime) {
 		return fmt.Errorf("slot of block is too high: %d", block.Slot)
 	}