The g:date-time type specifier represents the C GDateTime type which represents a date and time. 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.

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