diff --git a/doc/hg.1.txt b/doc/hg.1.txt
--- a/doc/hg.1.txt
+++ b/doc/hg.1.txt
@@ -127,6 +127,42 @@ SPECIFYING MULTIPLE REVISIONS
     A range acts as a closed interval.  This means that a range of 3:5
     gives 3, 4 and 5.  Similarly, a range of 4:2 gives 4, 3, and 2.
 
+DATE FORMATS
+------------
+
+    Some commands (backout, commit, tag) allow the user to specify a date.
+    Possible formats for dates are:
+
+YYYY-mm-dd \HH:MM[:SS] [(+|-)NNNN]::
+    This is a subset of ISO 8601, allowing just the recommended notations
+    for date and time. The last part represents the timezone; if omitted,
+    local time is assumed. Examples:
+
+    "2005-08-22 03:27 -0700"
+
+    "2006-04-19 21:39:51"
+
+aaa bbb dd HH:MM:SS YYYY [(+|-)NNNN]::
+    This is the date format used by the C library. Here, aaa stands for
+    abbreviated weekday name and bbb for abbreviated month name. The last
+    part represents the timezone; if omitted, local time is assumed.
+    Examples:
+
+    "Mon Aug 22 03:27:00 2005 -0700"
+
+    "Wed Apr 19 21:39:51 2006"
+
+unixtime offset::
+    This is the internal representation format for dates. unixtime is
+    the number of seconds since the epoch (1970-01-01 00:00 UTC). offset
+    is the offset of the local timezone, in seconds west of UTC (negative
+    if the timezone is east of UTC).
+    Examples:
+
+    "1124706420 25200" (2005-08-22 03:27:00 -0700)
+
+    "1145475591 -7200" (2006-04-19 21:39:51 +0200)
+
 ENVIRONMENT VARIABLES
 ---------------------