SDL times are signed, 64-bit integers representing nanoseconds since the Unix epoch (Jan 1, 1970).
Defined in <SDL3/SDL_stdinc.h>
typedef Sint64 SDL_Time;
#define SDL_MAX_TIME SDL_MAX_SINT64
#define SDL_MIN_TIME SDL_MIN_SINT64
They can be converted between POSIX time_t values with SDL_NS_TO_SECONDS() and SDL_SECONDS_TO_NS(), and between Windows FILETIME values with SDL_TimeToWindows() and SDL_TimeFromWindows().
This macro is available since SDL 3.1.3.