diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 9ed674f98..79855ebbc 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -975,6 +975,8 @@ def verify_merkle_branch(leaf: Hash32, branch: [Hash32], depth: int, index: int,
     return value == root
 ```
 
+Verify that `block.slot - (deposit_data.timestamp - state.genesis_time) // SLOT_DURATION < DELETION_PERIOD`.
+
 Run `add_validator(validators, deposit_data.deposit_params.pubkey, deposit_data.msg_value, deposit_data.deposit_params.proof_of_possession, deposit_data.deposit_params.withdrawal_credentials, deposit_data.deposit_params.randao_commitment, PENDING_ACTIVATION, block.slot)`.
 
 ## State recalculations (every `CYCLE_LENGTH` slots)