[Arjen Markus] (24 august 2005) The [[clock]] command is a wonderful instrument, despite all its quirks, if you need to do date/time computations. I ran into one quirk the other day that is not the fault of the implementation, but rather of the complicated calendar we use in today's world: ''daylight saving time''.

Let me explain my problem:
    * I had a starting date in october and I wanted to know the end date after N spring-neap cycles (in my approximation: 15 days and 6 hours).
    * I used the clock command to do the computation and got some date in january, 7 o'clock as the stop date/time.

Then I realised that I had crossed the date where the daylight saving time correction changes. 
By using the option '''-gmt 1''' you can avoid these complications.

Here is an illustration of the effect:

 # Daylight saving problems
 #
 # No care for daylight saving time corrections ...
 #
 set day1 [clock scan "2005-10-01"]
 set day2 [clock scan "2005-11-01"]
 set number_days [expr {($day2-$day1)/86400.0}]
 puts "Number of days: $number_days"

 # Option: -gmt 1
 #
 set day1 [clock scan "2005-10-01" -gmt 1]
 set day2 [clock scan "2005-11-01" -gmt 1]
 set number_days [expr {($day2-$day1)/86400.0}]
 puts "Number of days: $number_days"

The result (Tcl 8.4):

 Number of days: 31.0416666667
 Number of days: 31.0

Of course, it depends on what you want to achieve, if you need this option or not.
Date/time computations are simply very complicated.

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[LV] A friend sent me this pointer [http://sunsolve.sun.com/search/document.do?assetkey=1-26-102775-1] which discusses the fact that a number of countries have been, and continue to, make changes in the way [DST] is calculated.
Does anyone know whether Tcl is tracking these kinds of adjustments?

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[Category Date and Time] - [Category Example]