42.5 Time of Day

This section explains how to determine the current time and time zone.

Many functions like current-time and file-attributes return Lisp timestamp values that count seconds, and that can represent absolute time by counting seconds since the epoch of 1970-01-01 00:00:00 UTC (Coordinated Universal Time). Typically these counts ignore leap seconds; however, GNU and some other operating systems can be configured to count leap seconds.

Although traditionally Lisp timestamps were integer pairs, their form has evolved and programs ordinarily should not depend on the current default form. If your program needs a particular timestamp form, you can use the time-convert function to convert it to the needed form. See Time Conversion.

There are currently three forms of Lisp timestamps, each of which represents a number of seconds:

Function arguments, e.g., the time argument to format-time-string, accept a more-general time value format, which can be a Lisp timestamp, nil for the current time, a finite floating-point number for seconds, or a list (high low micro) or (high low) that is a truncated list timestamp with missing elements taken to be zero.

Time values can be converted to and from calendrical and other forms. Some of these conversions rely on operating system functions that limit the range of possible time values, and signal an error such as ‘"Specified time is not representable"’ if the limits are exceeded. For instance, a system might not support timestamps before the epoch, or years far in the future. You can convert a time value into a human-readable string using format-time-string, into a Lisp timestamp using time-convert, and into other forms using decode-time and float-time. These functions are described in the following sections.

Function: current-time-string &optional time zone

This function returns the current time and date as a human-readable string. The format does not vary for the initial part of the string, which contains the day of week, month, day of month, and time of day in that order: the number of characters used for these fields is always the same, although (unless you require English weekday or month abbreviations regardless of locale) it is typically more convenient to use format-time-string than to extract fields from the output of current-time-string, as the year might not have exactly four digits, and additional information may some day be added at the end.

The argument time, if given, specifies a time to format, instead of the current time. The optional argument zone defaults to the current time zone rule. See Time Zone Rules. The operating system limits the range of time and zone values.

     ⇒ "Fri Nov  1 15:59:49 2019"
Variable: current-time-list

This boolean variable is a transition aid. If t, current-time and related functions return timestamps in list form, typically (high low micro pico); otherwise, they use (ticks . hz) form. Currently this variable defaults to t, for behavior compatible with previous Emacs versions. Developers are encouraged to test timestamp-related code with this variable set to nil, as it will default to nil in a future Emacs version, and will be removed in some version after that.

Function: current-time

This function returns the current time as a Lisp timestamp. If current-time-list is nil, the timestamp has the form (ticks . hz) where ticks counts clock ticks and hz is the clock ticks per second. Otherwise, the timestamp has the list form (high low usec psec). You can use (time-convert nil t) or (time-convert nil 'list) to obtain a particular form regardless of the value of current-time-list. See Time Conversion.

Function: float-time &optional time

This function returns the current time as a floating-point number of seconds since the epoch. The optional argument time, if given, specifies a time to convert instead of the current time.

Warning: Since the result is floating point, it may not be exact. Do not use this function if precise time stamps are required. For example, on typical systems (float-time '(1 . 10)) displays as ‘0.1’ but is slightly greater than 1/10.

time-to-seconds is an alias for this function.

Function: current-cpu-time

Return the current CPU time along with its resolution. The return value is a pair (CPU-TICKS . TICKS-PER-SEC). The CPU-TICKS counter can wrap around, so values cannot be meaningfully compared if too much time has passed between them.