datetime: Use g_time_zone_new_identifier
g_time_zone_new() is deprecated in favour of g_time_zone_new_identifier().
g_time_zone_new_identifier() returns NULL if no matching identifier is found, so warn and fallback to UTC in that case.
g_time_zone_new() is deprecated in favour of g_time_zone_new_identifier().
g_time_zone_new_identifier() returns NULL if no matching identifier is found, so warn and fallback to UTC in that case.