Be more specific on when these functions fail.

man/khttp_datetime2epoch.3

@@ -66,10 +66,9 @@ If
 is
 .Dv NULL ,
 simply check the time values for correctness.
-Date correctness means valid time values, valid day of month value given
-month and leap year, and valid month value.
 .Pp
-The date and time arguments are as follows:
+Date correctness means that input values fall within the following
+acceptable ranges:
 .Bl -column hourx aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -offset indent
 .It Fa sec Ta seconds 0\(en59
 .It Fa min Ta minutes 0\(en59

man/khttp_epoch2tms.3

@@ -93,8 +93,11 @@ For conversion without possible overflow or underflow, use
 .Xr khttp_epoch2datetime 3 .
 .Sh RETURN VALUES
 These return zero on failure and non-zero on success.
-Failure occurs if the computed year will overflow or underflow
-.Fa tm_year .
+Failure occurs if
+.Fa epoch
+will overflow or underflow
+.Fa tm_year ,
+which occurs around the year \(+-6.7+16.
 .Sh SEE ALSO
 .Xr khttp_datetime2epoch 3 ,
 .Xr khttp_epoch2datetime 3 ,