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Re: Time Woes [message #30548 is a reply to message #30471]

Thu, 02 May 2002 14:51

thompson
Messages: 584
Registered: August 1991

Senior Member

btupper@bigelow.REMOVE.org (Ben Tupper) writes:

> On Wed, 1 May 2002 13:41:10 -0600, David Fanning <david@dfanning.com>
> wrote:

>> I have a time array that purports to be the number of seconds
>> from 1 Jan 1980. How in the world do I convert that to something
>> I can use with routines I have access to?
>>

> Howdy,

> Do you mean that you want to know a Julian time stamp or Calendar date
> for the time array values? I don't have IDL in front of me right now
> so I can't check this, and I can't clearly recall the syntax.

> (The following supposes the 'seconds' are just the run-of-the-mill
> kind of seconds...)

> ; start time in seconds
> t0 = JulDay( 0,1 1980, 0, 0, 0)
> ; add the elapsed time since t0
> t[i] = t0 + timeArray[i]
> ;convert to calendar values
> CalDat, t[i], month, day, year, hour, min, sec

> I have a sneaky feeling that your problem is more complicated than
> this solution will bridge. The only thing I do well with time (in
> IDL) is squander it, but I hope it helps anyway.

There is an ambiguity in the problem. It isn't obvious whether the seconds in
timeArray include leapseconds or not. In other words, is it a UTC-based time,
or a TAI-based time? The difference in the two between 1-Jan-1980 and today is
13 seconds. Depending on your point of view that could be negligable or a
major mistake.

My gut feeling is that your data is UTC-based. This would make it comparable
to Unix time, which is the number of (non-leap) seconds since 1-Jan-1970, and
which in IDL is returned with SYSTIME(1). In that case, using the JulDay and
CalDat procedures would give you exactly the correct answers.

If on the other hand, the data you have is TAI-based, and you care about the
distinction, you might want to check out my routines utc2tai.pro and
tai2utc.pro, which are available at

ftp://sohoftp.nascom.nasa.gov/solarsoft/gen/idl/time/

William Thompson

P.S. By the way, wouldn't that be

; start time in seconds
t0 = JulDay( 1,1, 1980, 0, 0, 0) * 86400.D0

;^)

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[Message index]

		Time Woes By: David Fanning on Wed, 01 May 2002 12:41
		Re: Time Woes By: Craig Markwardt on Thu, 02 May 2002 15:26
		Re: Time Woes By: thompson on Thu, 02 May 2002 14:51
		Re: Time Woes By: btupper on Thu, 09 May 2002 07:21
		CDF_EPOCH (was Re: Time Woes) By: Brian Jackel on Thu, 02 May 2002 12:05
		Re: Time Woes By: Struan Gray on Thu, 02 May 2002 08:34
		Re: Time Woes By: David Fanning on Thu, 02 May 2002 06:38
		Re: Time Woes By: btupper on Thu, 02 May 2002 06:11
		Re: Time Woes By: Ken Mankoff on Thu, 02 May 2002 06:07
		Re: Time Woes By: Paul Van Delst[1] on Wed, 01 May 2002 14:24
		Re: Time Woes By: David Fanning on Wed, 01 May 2002 14:00
		Re: Time Woes By: Robert Stockwell on Mon, 06 May 2002 06:14
		Re: Time Woes By: Ken Mankoff on Wed, 01 May 2002 13:10

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