[fpc-pascal] convert "epoch" to UTC

[waldo kitty]

i'm needing to convert a "UTC epoch" date to a "standard time string"... by 
that, i mean that i want to get something like "2011-01-13 22:24:04" out of the 
"epoch" number 12013.9338171...

the above "epoch" number is (supposedly) built like this...

12 == 2012
013 == 13th day of the year
.9338171 * 24 == 22.4116104 hours
.4114104 * 60 == 24.696624 minutes

and here's where it breaks on me because

.696624 * 60 == 41.79744 seconds and that doesn't match the above time string 
showing 04 seconds... that time string was built from this "epoch" number (by 
another program) and i assume that it followed this published formula for this 
format (which is for satellite TLEs)...

so, my question is this... is there an existing FPC routine that will convert 
this (or a part of this) properly or do i need to play with something to get it 
right? it is obvious that this is not a "unix epoch"... i'm not sure how 
"normal" julian date epochs are represented...

i've looked through the docs on freepascal.org but none of them show an number 
with which to compare... there's no links pointing to a definition or example of 
a julian date or unix date or any such...

help! please :/