clock_t and time_t are arithmetic types capable of representing times.

[section 6.2.5 Types, pdf pages 46-47, defines an arithmetic type as an integer
or floating type; and a floating type is defined as a real floating type or
complex type.  So clock_t and time_t could be floating or complex, but on
Unix systems they are an integer type, and a time_t value represents the
number of seconds elapsed since the Epoch, 1970-01-01 00:00:00 +0000 (UTC)]

CLOCKS_PER_SEC expands to an expression with type clock_t that is
the number per second of the value returned by the clock function.

  clock_t clock(void);			  // determines the processor time used

  double difftime(time_t t1, time_t t0);  // time difference in seconds

  time_t mktime(struct tm *tmptr);	  // create time_t from struct tm

  time_t time(time_t *timer);		  // current calendar time

Time conversion functions:

  char *asctime(const struct tm *tmptr);  // produces a string like "Sun Sep 16 01:03:52 1973\n"

  char *ctime(const time_t *timer);	  // same as asctime(localtime(timer))

  struct tm *gmtime(const time_t *timer); // create struct tm from time_t

  struct tm *localtime(const time_t *timer);

  size_t strftime(char * restrict s, size_t maxsize,
    const char * restrict format, const struct tm * restrict timeptr);

The tm structure shall contain at least the following members, in any order.
The semantics of the members and their normal ranges are expressed in the
comments.274)

  int tm_sec;	// seconds after the minute - [0, 60]
  int tm_min;	// minutes after the hour - [0, 59]
  int tm_hour;	// hours since midnight - [0, 23]
  int tm_mday;	// day of the month - [1, 31]
  int tm_mon;	// months since January - [0, 11]
  int tm_year;	// years since 1900
  int tm_wday;	// days since Sunday - [0, 6]
  int tm_yday;	// days since January 1 - [0, 365]
  int tm_isdst;	// Daylight Saving Time flag

274) The range [0, 60] for tm_sec allows for a positive leap second.

The value of tm_isdst is positive if Daylight Saving Time is in effect,
zero if Daylight Saving Time is not in effect, and negative if the
information is not available.

276) Thus, a positive or zero value for tm_isdst causes the mktime function
to presume initially that Daylight Saving Time, respectively, is or is not
in effect for the specified time. A negative value causes it to attempt to
determine whether Daylight Saving Time is in effect for the specified time.