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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. [6] Unix time has historically been encoded as a signed 32-bit integer, a data type composed of 32 binary digits (bits) which represent an ...
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 ...
The Network Time Protocol used to coordinate time between computers uses an epoch of 1 January 1900, counted in an unsigned 32-bit integer for seconds and another unsigned 32-bit integer for fractional seconds, which rolls over every 2 32 seconds (about once every 136 years).
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
System time can be converted into calendar time, which is a form more suitable for human comprehension. For example, the Unix system time 1 000 000 000 seconds since the beginning of the epoch translates into the calendar time 9 September 2001 01:46:40 UT.
Bits 45–48 encode tenths of seconds (0–9) Bits 50–53 encode years, and bits 55–58 encode tens of years (0–99) Bits 80–88 and 90–97 encode "straight binary seconds" since 00:00 on the current day (0–86399, not BCD) In IRIG G, bits 50–53 encode hundredths of seconds, and the years are encoded in bits 60–68.
TT differs from Geocentric Coordinate Time (TCG) by a constant rate. Formally it is defined by the equation = +, where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, is the constant difference in the rates of the two time scales, and is a constant to resolve the epochs (see below).
The 64-bit timestamps used by NTP consist of a 32-bit part for seconds and a 32-bit part for fractional second, giving NTP a time scale that rolls over every 2 32 seconds (136 years) and a theoretical resolution of 2 −32 second (233 picoseconds). NTP uses an epoch of 1 January 1900.