See http://purl.org/tcl/home/man/tcl8.4/TclCmd/clock.htm for the formal man page.


Example:

 %clock format [clock seconds] -format %Y-%m-%dT%H:%M:%S

returns an ISO8601 [http://en.wikipedia.org/wiki/Iso8601] formatted timestamp, such as:

 2006-06-24T09:48:29

[WJP] 2007-10-20

A conversion specifier added (but absent from the manual page) in late versions of 8.4 provides the timezone offset. The full ISO8601 timestamp is therefore

 clock format [clock seconds] -format %Y-%m-%dT%H:%M:%S%z

which produces, e.g.:

 2007-10-20T21:56:51-0700

----
[JMN] 2006-06-24 

[[clock format]] doesn't directly allow you to produce an ISO8601 timestamp that includes fractional seconds - you have to tack that on yourself because [[clock format]] expects an integer number of seconds. I guess you can just grab the last 3 digits of [[clock milliseconds]] and tack them on with the '.' separator.

----

Recent implementations (e.g Tcl 8.5a4) of [[clock format]] show a fivefold slow-down in comparison to Tcl 8.4.

e.g now roughly 0.5 milliseconds per [[clock format]] on an old 400MHZ machine

While I understand this is due to the long overdue [Reworking the clock command] - and the current implementation is pure Tcl.. Can we expect an improvement of the performance of this?

[[clock format]] in Tcl8.5 now accepts extra options such as -timezone.
See http://purl.org/tcl/home/man/tcl8.5/TclCmd/clock.htm for the formal man page.

I think the performance issue is actually quite important. Many systems use timestamps extensively - and also need to extract fields (year,minute etc) relatively quickly. e.g Logging systems, mail handling protocols, certain object-oriented models.

The performance is even worse on the first [[clock format]] call in each interp.
Approximately 800 milliseconds. I guess it's loading timezone information from disk?
This delay is *terrible* for the startup time of a multithreaded or even just multi-interpreter app.

If all this extra timezone stuff is going to continue cost us in terms of performance, would it make sense to have a separate command for fast extraction of basic timestamp fields - without the complication of timezones? i.e treat the provided timestamp as TAI and the caller can use [[expr]] to add the relevant timezone offset before formatting if they need to.
(Maybe some other command could then be made available to calculate offsets that account for timezones and leap-seconds?)

e.g something like

 [[clock fields [clock milliseconds] -resolution milliseconds]

returning a minimal fixed order list suitable for use by [[array set]] or [[lindex]], [[lassign]] etc.

 {year 2006 month 06 day 24 hour 09 min 48 sec 29 millisec 11} 

----
See also: [clock] & Parsing ISO8601 dates and times [http://wiki.tcl.tk/13094]
----

"... does anyone know why [clock format] defaults to using the root locale instead of the current locale?"

"Yes. Far more uses of the formatted time are read by programs than by people."
----
[[
[Category Command]
|
[Category Date and Time]
]]