11.2 Overview of Date/Time Functions

In the UTC-based model, times are represented as seconds since the Epoch. The Epoch is defined as the time 0 hours, 0 minutes, 0 seconds, January 1, 1970 UTC. Seconds since the Epoch is a value interpreted as the number of seconds between a specified time and the Epoch.

The functions time and ftime return the time as seconds since the Epoch.

The functions ctime, gmtime, and localtime take as their argument a time value that represents the time in seconds from the Epoch.

The function mktime converts a broken-down time, expressed as local time, into a time value in terms of seconds since the Epoch.

The values st_ctime, st_atime, and st_mtime returned in the stat structure by the stat and fstat functions are also in terms of UTC.

Time support new to OpenVMS Version 7.0 includes the functions tzset, gettimeofday, and getclock, and the external variables tzname, timezone and daylight.

The UTC-based time model enables the DEC C RTL to:

• Implement the ANSI C gmtime function, which returns a structure in terms of GMT time.

• Specify the ANSI tm_isdst field of the tm structure, which specifies whether daylight-savings time is in effect.

• Provide time-related POSIX and X/Open extensions (such as the tzset function (which lets you get time information from any timezone), and the external variables tzname, timezone, and daylight.

• Correctly compute the local time for times in the past, something that the time functions like localtime need to do.

• Enable localtime and gmtime, through the use of feature- test macros (see Section 1.5), to return two additional fields: tm_zone (an abbreviation of the timezone name) and tm_gmtoff (the offset from UTC in seconds) in the tm structure they return.