diff --git a/ChangeLog b/ChangeLog
index 94f12c0..5b92cd5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -14,6 +14,8 @@ UNRELEASED Version 0.9.0
       - Raise error when ARC signing if i= instance limit value of 50 is
         exceeded
       - Specified that for ARC, Authentication-Results should not be signed
+      - Added missing documentation for timestamp function dkim.arc_sign
+        (LP: #1800314)
     - Fix DNS lookups to be compatible with EAI addresses in domains and
       selectors (John Levine)
     - Add type Hinting for sign and verify functions (LP: #1782596)
diff --git a/dkim/__init__.py b/dkim/__init__.py
index 2ff96fd..395cd64 100644
--- a/dkim/__init__.py
+++ b/dkim/__init__.py
@@ -1265,6 +1265,7 @@ def arc_sign(message, selector, domain, privkey,
     @param srv_id: the authserv_id used to identify the ADMD's AR headers
     @param signature_algorithm: the signing algorithm to use when signing
     @param include_headers: a list of strings indicating which headers are to be signed (default all headers not listed as SHOULD NOT sign)
+    @param timestamp: the time in integer seconds when the message is sealed (default is int(time.time) based on platform, can be string or int)
     @param logger: a logger to which debug info will be written (default None)
     @return: A list containing the ARC set of header fields for the next instance
     @raise DKIMException: when the message, include_headers, or key are badly formed.