After line 82902 add:

    %s   Seconds since 1970-01-01 00:00:00 UTC as a decimal number
         (see [xref to XBD 4.15 Seconds Since the Epoch])

Cross-volume changes to XSH:

After page 2009 line 63596 section strftime, add:

[CX]s    Replaced by the number of seconds since 1970-01-01 00:00:00 UTC
         as a decimal number (see [xref to XBD 4.15 Seconds Since the
         Epoch]). [all members, as if by a call to mktime()] [/CX]

After page 2027 line 64161 section strptime, add:

    s    The number of seconds since 1970-01-01 00:00:00 UTC as a
         decimal number (see [xref to XBD 4.15 Seconds Since the
         Epoch]); leading zeros shall be permitted but shall not be
         required.

Cross-volume changes to XSH:

After page 2009 line 63596 section strftime, add:

[CX]s    Replaced by the number of seconds since the Epoch
         as a decimal number (see [xref to XBD 4.15 Seconds Since the
         Epoch]). [all members, as if by a call to mktime()] [/CX]

At page 2011 line 63686 section strftime(), change:
        Otherwise, 0 shall be returned and the contents of the array are
        unspecified.
to:
        Otherwise, 0 shall be returned, <CX>errno shall be set to indicate
        the error,</CX> and the contents of the array are unspecified.

At page 2011 line 63689 section strftime(), replace the entire ERRORS section with:
       <CX>These functions shall fail if:

        [EOVERFLOW]   The format string includes a %s conversion and the
                     number of seconds since the Epoch cannot be represented
                     in a time_t.

        These functions may fail if:

        [EINVAL]   The format string includes a %s conversion and the number of
                   seconds since the Epoch would be negative.</CX>

After page 2027 line 64161 section strptime, add:

    s    The number of seconds since the Epoch as a
         decimal number (see [xref to XBD 4.15 Seconds Since the
         Epoch]); leading zeros shall be permitted but shall not be
         required.