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Software timekeeping systems vary widely in the resolution of time measurement; some systems may use time units as large as a day, while others may use nanoseconds.For example, for an epoch date of midnight UTC (00:00) on 1 January 1900, and a time unit of a second, the time of the midnight (24:00) between 1 January 1900 and 2 January 1900 is represented by the number 86400, the number of ...
computes the difference in seconds between two time_t values time: returns the current time of the system as a time_t value, number of seconds, (which is usually time since an epoch, typically the Unix epoch). The value of the epoch is operating system dependent; 1900 and 1970 are often used. See RFC 868. clock
Unix time [a] is a date and time representation widely used in computing. It measures time by the number of non-leap seconds that have elapsed since 00:00:00 UTC on 1 January 1970, the Unix epoch. For example, at midnight on 1 January 2010, Unix time was 1262304000. Unix time originated as the system time of Unix operating systems.
System time is measured by a system clock, which is typically implemented as a simple count of the number of ticks that have transpired since some arbitrary starting date, called the epoch. For example, Unix and POSIX -compliant systems encode system time (" Unix time ") as the number of seconds elapsed since the start of the Unix epoch at 1 ...
[a] For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2 451 545. [7] The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. Julian dates are expressed as a Julian day number with a decimal fraction added. [8]
There may also be additional parts, such as the day of the week. Years are usually counted from a particular starting point, usually called the epoch, with era referring to the span of time since that epoch. [b] A date without the year may also be referred to as a date or calendar date (such as "28 January" rather than "28 January 2025"). As ...
A distinction is sometimes made between the terms datestamp, timestamp and date-timestamp: Datestamp or DS: A date, for example 2025-01-25 according to ISO 8601; Timestamp or TS: A time of day, for example 05:00:22 using 24-hour clock; Date-timestamp or DTS: Date and time, for example 2025-01-25, 05:00:22
Many computer systems measure time and date using Unix time, an international standard for digital timekeeping.Unix time is defined as the number of seconds elapsed since 00:00:00 UTC on 1 January 1970 (an arbitrarily chosen time based on the creation of the first Unix system), which has been dubbed the Unix epoch.