- Time t
The time_t datatype is a
data type in the ISO C library defined for storingsystem time values. Such values are returned from the standard time() library function. This type is atypedef defined in the standard <time.h> header.ISO C defines time_t as an arithmetic type, but does not specify any particular type, range, resolution, or encoding for it. Also unspecified are the meanings of arithmetic operations applied to time values.Unix and POSIX-compliant systems implement the time_t type as a signed integer (typically 32 or 64bits wide) which represents the number of seconds since the start of theUnix epoch :midnight UTC ofJanuary 1 1970 (not countingleap seconds ). Some systems support negative time values, while others do not. 32 bit time_t is deprecated, due to theYear 2038 problem . [ [http://pw1.netcom.com/~rogermw/Y2038.html The Year 2038 problem] , Roger M. Wilcox. Retrieved on2008 -05-19.]In addition to the time() function, ISO C also specifies other functions and types for converting time_t
system time values into calendar times and vice versa.Example
The following C code retrieves the current time, formats it as a string, and writes it to the standard output.
#include
#include/*
* The result should look something like
* Fri 2008-08-22 15:21:59 WAST
*/int main(void){ time_t now; struct tm *ts; char buf [80] ;
// Get the current time now = time(NULL);
// Format and print the time, "ddd yyyy-mm-dd hh:mm:ss zzz" ts = localtime(&now); strftime(buf, sizeof(buf), "%a %Y-%m-%d %H:%M:%S %Z", ts); printf("%s ", buf);
return 0;}
time_t parties
Unix enthusiasts have a history of holding time_t parties to celebrate significant values of the Unix time number. [cite web |url=http://slashdot.org/articles/05/03/17/169200.shtml?tid=130 |title=Slashdot ! date +%s Turning 1111111111 |accessdate=2007-07-14 |format= |work= ] These are directly analogous to the
new year celebrations that occur at the change of year in many calendars. As the use of Unix time has spread, so has the practice of celebrating its milestones. Usually it is time values that are round numbers indecimal that are celebrated, following the Unix convention of viewing time_t values in decimal. Among some groups round binary numbers are also celebrated, such as +230 which occurred at 13:37:04UTC onJanuary 10 ,2004 .The events that these celebrate are typically described as "N seconds since the Unix epoch", but this is inaccurate. As discussed above, due to the handling of
leap second s in Unix time, the number of seconds elapsed since the Unix epoch is slightly greater than the Unix time number, for times later than the epoch.At 01:46:40
UTC onSeptember 9 ,2001 , theUnix billennium (Unix time number 1000000000) was celebrated.At 01:58:31
UTC onMarch 18 ,2005 , the Unix time number reached 1111111111.At 23:31:30
UTC onFebruary 13 ,2009 , a celebration is expected as the Unix time number reaches 1234567890 seconds. This day happens to fall onFriday the 13th on theGregorian calendar . The UNIX time number 0x50000000 (1342177280 seconds) will also happen on aFriday the 13th , at 11:01:20UTC onJuly 13 ,2012 .At 03:33:20
UTC onMay 18 2033 , the second billennium will be celebrated (Unix time number 2000000000).ee also
*
C standard library
*Real-time clock
*System time
*Unix time
*Year 2038 problem References
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