Tcl Tutorial Lesson 41

Time and Date - clock

The clock command provides access to the time and date functions in Tcl. Depending on the subcommands invoked, it can acquire the current time, or convert between different representations of time and date.

clock seconds
The clock seconds command returns the time in seconds since the epoch. The date of the epoch varies for different operating systems, thus this value is useful for comparison purposes, or as an input to the clock format command.
clock format clockValue ?-gmt boolean? ?-format string? ?-timezone zone?
The format subcommand formats a clockValue (as returned by clock seconds) into a human readable string. The -gmt switch takes a boolean as the second argument. If the boolean is 1 or true, then the time will be formatted as Greenwich Mean Time (in particular: no daylight saving), otherwise, it will be formatted as local time, taking care of daylight saving time and timezones.

The -format option controls in what format the date/time will be in. The contents of the string argument to format has similar contents as the format statement. However, the %* descriptors are dedicated to dates and times:

%aAbbreviated weekday name (Mon, Tue, etc.)
%AFull weekday name (Monday, Tuesday, etc.)
%bAbbreviated month name (Jan, Feb, etc.)
%BFull month name (January, February, etc.)
%dDay of month
%jJulian day of year
%mMonth number (01-12)
%yYear in century
%YYear with 4 digits
%HHour (00-23)
%IHour (00-12)
%MMinutes (00-59)
%pPM or AM
%DDate as %m/%d/%y
%rTime as %I:%M:%S %p
%RTime as %I:%M
%TTime as %I:%M:%S
%ZTime Zone Name
clock scan dateString ?-format format?
The scan subcommand converts a human-readable string to a system clock value, as would be returned by clock seconds.

The dateString argument for the clock scan command, without the -format format option, may be a string in any of these forms:

  • time: A time of day in one of the formats shown below. Meridian may be AM, or PM, or a capitalization variant. If it is not specified, then the hour (hh) is interpreted as a 24 hour clock. Zone may be a three letter description of a time zone, EST, PDT, etc.
    • hh:mm:ss ?meridian? ?zone?
    • hhmm ?meridian? ?zone?
  • date: A date in one of the formats shown below.
    • mm/dd/yy
    • mm/dd
    • monthname dd, yy
    • monthname dd
    • dd monthname yy
    • dd monthname
    • day, dd monthname yy

This is known as "free form scanning". The clock scan command then uses a number of heuristic rules to determine the correct form. However, it is safer to explicitly specify a format to which the date/time string should conform. This way you can be sure that the date/time is interpreted in a controlled way.

A small example of the use of the -format option with clock scan: in German speaking countries, the date is often formatted as 2.1.2017, meaning the second of January, 2017. This is not in a form accepted by the free form scanning rules, so an explicit format is required:

   % puts [clock scan "2017.01.02" -format "%Y.%m.%d"]
   % puts [clock format 1483311600]
   Mon Jan 02 00:00:00 CET 2017

Arithmetic with the clock command is also possible:

   # Calculate the date ten days and 2 hours from today
   set today [clock seconds]

   set 10daysFromNow [clock add $today 10 days 2 hours]
   puts [clock format $today]
   puts [clock format $10daysFromNow]

might print:

Thu Jun 29 19:48:27 CEST 2017
Sun Jul 09 21:48:27 CEST 2017

(Note the time zone: the dates fall within the daylight saving period of the year 2017)

Other subcommands of the clock command are useful to measure time in shorter units than a second: clock milliseconds and clock microseconds.


set Time [clock seconds]

puts "The time is: [clock format $Time -format %H:%M:%S]"
puts "The date is: [clock format $Time -format %D]"
puts [clock format $Time -format {Today is: %A, the %d of %B, %Y}]
puts "The default format for the time is: [clock format $Time]\n"

set halBirthBook "Jan 12, 1997"
set halBirthMovie "Jan 12, 1992"
set bookSec [clock scan $halBirthBook]
set movieSec [clock scan $halBirthMovie]

puts "The book and movie versions of '2001, A Space Odyssey' had"
puts "a discrepancy of [expr {$bookSec - $movieSec}] seconds"
puts "in how soon sentient computers like the HAL 9000"
puts "would appear"

  Resulting output
The time is: 19:51:19
The date is: 06/29/2017
Today is: Thursday, the 29 of June, 2017
The default format for the time is: Thu Jun 29 19:51:19 CEST 2017

The book and movie versions of '2001, A Space Odyssey' had
a discrepancy of 157852800 seconds
in how soon sentient computers like the HAL 9000
would appear