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Re: days of the week [message #29867 is a reply to message #29866]

Wed, 20 March 2002 08:26

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

David Fanning <david@dfanning.com> writes:

> Bastienne Schneiter (b.schneiter@meteonews.ch) writes:
>
>> I need to show a date in a little application. Its format is
>> YYYYMMDDHH. But I also have to show to which day of the week this date
>> correspond (MON, TUE etc).
>> Does anybody know how to determine the day of the week for a given
>> date?
>
> Here is a bonus question. What is the significance
> of a meteorological event that happens on a Thursday?

I think you mean an "astronomical" or "celestial" event, and it
actually happens today at 19:16:08 UTC.

To answer Bastienne's original question, it's actually easier to
compute the day of the week than the calendar date. That's because
there are no "leap" weekdays, or irregularly sized weeks. Weeks are
always exactly seven days long.

We know that March 24, 2002, is a Sunday. Thus, we can compute the
day of the week using the MOD function:

dayweek = (julday(month,day,year) - julday(3,24,2002)) MOD 7
dayweek = (dayweek + 7) MOD 7

Where 0 means Sunday and 6 means Saturday. The second statement is to
handle cases when DAYWEEK is negative, which happens with the MOD
function unfortunately.

Craig

P.S. I think the built-in JULDAY function is the one of the most
dangerous function I have ever seen. It measures *calendar dates*
from *noon* by gosh! It also doesn't handle fractional days, which is
a pity, but thankfully there are tons of IDL Astro/JHU functions which
overcome this.

--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------

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[Message index]

		Re: days of the week By: Craig Markwardt on Thu, 21 March 2002 09:01
		Re: days of the week By: Craig Markwardt on Thu, 21 March 2002 09:01
		Re: days of the week By: Doug Rowland on Thu, 21 March 2002 07:34
		Re: days of the week By: the_cacc on Thu, 21 March 2002 03:21
		Re: days of the week By: Mark Hadfield on Wed, 20 March 2002 14:20
		Re: days of the week By: Mark Hadfield on Wed, 20 March 2002 14:26
		Re: days of the week By: Ed Wright on Wed, 20 March 2002 09:17
		Re: days of the week By: Randall Skelton on Wed, 20 March 2002 09:42
		Re: days of the week By: David Fanning on Wed, 20 March 2002 08:59
		Re: days of the week By: David Fanning on Wed, 20 March 2002 08:49
		Re: days of the week By: eddie haskell on Wed, 20 March 2002 08:25
		Re: days of the week By: Craig Markwardt on Wed, 20 March 2002 08:26
		Re: days of the week By: David Fanning on Wed, 20 March 2002 07:47
		Re: days of the week By: David Fanning on Wed, 20 March 2002 07:12
		Re: days of the week By: Martin Downing on Mon, 25 March 2002 09:07

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