The g:date-time type specifier performs automatic conversion between the GDateTime time representation and the Lisp universal time, that is measured as an offset from the beginning of the year 1900. The Lisp implementation uses the g_date_time_new_from_unix_utc() and g_date_time_to_unix() functions for conversion. The g:date-time type specifier takes into account the offset between the Unix time that is the number of seconds that have elapsed since 1970-01-01 00:00:00 UTC and the Lisp universal time that is counted from 1900-01-01 00:00:00 UTC.

(cffi:convert-to-foreign 0 'g:date-time)
=> #.(SB-SYS:INT-SAP #X558E3298CE40)
(cffi:convert-from-foreign * 'g:date-time) => 0