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.

The clock command is a platform independent method of getting the display functionality of the unix date command, and provides access to the values returned by a unix gettime() call.

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?
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, otherwise, it will be formatted as local time.

The -format option controls what format the return will be in. The contents of the string argument to format has similar contents as the format statement (as discussed in lesson 19, 33 and 34). In addition, there are several more %* descriptors that can be used to describe the output.

These include:

clock scan dateString -option value...?
The scan subcommand converts a human readable string to a system clock value, as would be returned by clock seconds

The -format option is used to describe the format of the dateString

If -format is not used, the command tries to guess the format of dateString, sometimes with surprising results. It's best to use -format. The following forms will probably return expected results:

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?
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


set systemTime [clock seconds]

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

set halBirthBook "Jan 12, 1997"
set halBirthMovie "Jan 12, 1992"
set bookSeconds [clock scan $halBirthBook -format {%b %d, %Y}]
set movieSeconds [set movieSeconds [clock scan $halBirthMovie -format {%b %d, %Y}]]

puts "The book and movie versions of '2001, A Space Oddysey' had a"
puts "discrepancy of [expr {$bookSeconds - $movieSeconds}] seconds in how"
puts "soon we would have sentient computers like the HAL 9000"