diff --git a/bip-0138.md b/bip-0138.md
index 555bb3de..e8974260 100644
--- a/bip-0138.md
+++ b/bip-0138.md
@@ -487,6 +487,8 @@ descriptor set, with `start <= end`.
 `birth_time` is an optional integer Unix timestamp in seconds, indicating a
 lower bound for when the descriptor set may have received funds. Importing
 wallets MAY use this value as a scanning hint.
+If `0` or omitted, no lower bound is known. Importers SHOULD scan from the wallet
+birthday if known, otherwise from genesis.
 
 #### BIP388 Wallet Policy Backup Content
 
@@ -529,6 +531,8 @@ policy, with `start <= end`.
 `birth_time` is an optional integer Unix timestamp in seconds, indicating a lower
 bound for when the policy may have received funds. Importing wallets MAY use this
 value as a scanning hint.
+If `0` or omitted, no lower bound is known. Importers SHOULD scan from the wallet
+birthday if known, otherwise from genesis.
 
 ##### Multiple Policies Document