[fpc-pascal] convert "epoch" to UTC

[Sven Barth]

Am 16.01.2012 23:57, schrieb waldo kitty:
> anyway, the format is like this...
>
> *_TLE Epoch Format_*
>
> eg: 12013.93338171
>
> 1. there are always 5 digits to the left of the decimal. the first two
> digits
> are the year of the epoch.
>
> if year<57 then year:=year+2000 else year:=year+1900
>
> in the above, 12 is the year which is 2012.
>
> 2. the next three digits (up to the decimal point) are the "day number
> of the
> year" of the TLE Epoch record.
>
> in the above, 013 is the day number of the year. feed this plus the year
> to a julian date routine to find the julian date number. in this example,
> we're looking for the julian date number for the 13th day of 2012.
>
> 3. the digits from the decimal point to the end are the fractional part of
> the day. these are /not/ in julian date or julian day format.
>
> in the above, .93338171 is the fractional part of the day. the time portion
> of the formula goes like this.
>
> fooo := frac(12013.93338171)
> rslt := fooo * 24 // 22.40116104
> hour := trunc(rslt) // 22
> fooo := frac(rslt) // .40116104
> rslt := fooo * 60 // 24.0696624
> mins := trunc(rslt) // 24
> fooo := frac(rslt) // .0696624
> rslt := fooo * 60 // 4.179744
> secs := trunc(rslt) // 4
> fooo := frac(rslt) // .179744
> rslt := fooo * 1000 // the more zeros, the more precision
> msec := trunc(rslt) // 179
>
> 4. convert the julian date number from #2 to TDateTime. fill in the
> TDateTime
> time fields from #3. now EncodeDateTime and you'll have the proper numbers
> in a TDateTime record.
>
> so what i ran into, besides the julian date routines not doing the time,
> but i was also sending the entire decimal number, without the year
> digits, to the julian date routine and that was just flat wrong since
> the fractional side wasn't a julian date fraction! ::TripleFacePalm::
>
> so there's three pieces of data stored in that one "TLE Epoch" number
> and each has to be broken out and treated differently... now my answers
> finally agree with the programs i've been attempting to verify them
> against... i just couldn't see the forest for the trees and couldn't see
> the trees for all the leaves on'em :LOL:

Have you tried just adding the fractional part (which you called "fooo" 
above) to the TDateTime value returned by TryJulianToDateTime and then 
feeding that to FormatDatTime? It should give you the same result... (at 
least I hope that ^^)

Regards,
Sven