Here is the answer I found in the mono source code.
void _wapi_time_t_to_filetime (time_t timeval, WapiFileTime *filetime) { guint64 ticks; ticks = ((guint64)timeval * 10000000) + 116444736000000000ULL; filetime->dwLowDateTime = ticks & 0xFFFFFFFF; filetime->dwHighDateTime = ticks >> 32; }