| Re: hours since 1-1-1 [message #44864 is a reply to message #44859] |
Tue, 26 July 2005 20:00   |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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qian wrote:
> Dear IDL users:
>
> Is there an IDL function/program that converts the unidata time unit to
> more readable format?
>
> For example: hours since 1-1-1 17338824 -> Jan 1 1979.
Oh no, not hours since 1-1-1 again! That must be the silliest ever
date-time origin ever invented for modern data! I mean, what calendar
were they using then and why should I have to care?
Anyway, IDL has the JULDAY function, which calculates time in Julian
days, ie time since 12:00 hours on 1 Jan 4713BC. (OK, I withdraw my
comment about 1 Jan 0001 being the silliest ever date-time origin.)
The confusing things you have to remember:
- JULDAY uses the weird month, day, year order for dates.
- The Julian date reaches an integral at 12:00 each day, not at
00:00.
- JULDAY accepts optional hour, minute, second arguments. With them
it returns the true Julian date-time as a double precision floating
point number. Without them it returns the Julian date to be reached
at 12:00 on the day in question as an integer. (Got that?)
Consider the following calculations
IDL> print, julday(1,1,1,0,0,0)
1721423.5
IDL> print, julday(1,1,1979,0,0,0)-julday(1,1,1,0,0,0)
722451.00
IDL> print, 24*(julday(1,1,1979,0,0,0)-julday(1,1,1,0,0,0))
17338824.
It seems that JULDAY agrees with you and with Unidata, that
00:00 on 1 Jan 1979 was 722451 days == 17338824 hours after 00:00 on 1
Jan 0001.
IDL also offers the CALDAT function, to convert Julian date-times to
calendar date-times and the calendar format codes that let it express
real numbers as date-time strings (on the assumption they are Julian
date-times).
So to deal with "hours after 1-1-1" in IDl convert them to Julian
date-times immediately (divide by 24 then add 1721423.5D0) and then use
IDL's built-in facilities thereafter.
--
Mark Hadfield "Kei puwaha te tai nei, Hoea tahi tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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