What are clock_t and time_t?

[jamadagni]

In the description of time.h I have read that clock_t and time_t are arithmetic types representing times, but I do not understand what exact datatypes these are - how many bytes they hold, etc. For instance, if I have:

#include <time.h>
time_t now;
now = time();

#include <time.h>
time_t now;
now = time();

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what formatting would I need to use if I use the variable now in a printf statement?