I want to make my code more platform-/implementation-independent. I don't know what a time_t will be implemented as on the platform when the code is being compiled. How do I know the type of t to determine what format specifier to use?
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time_t t = time(NULL);
printf("%s", t);
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Generally, the way to display the value of a time_t is to break down its components to a struct tm using gmtime or localtime and display those or convert them as desired with strftime, or ctime to go directly from time_t to a string showing local time.
If you want to see the raw value for some purpose, the C standard specifies that time_t is real, which means it is integer or floating-point (C 2011 (N1570) 6.2.5 17). Therefore, you should be able to convert it to double and print that. There is some possibility that time_t can represent values that double cannot, so you might have to guard against that if you want to take care regarding exotic implementations. Since difftime returns the difference of two time_t objects as a double, it seems C does not truly support time_t with more precision than a double.