Dave Ballentine showed you step by step how the conversion is going to work; but if you use the final answer, you don't get the timne portion, just the correct date;

you lose the modulus of the integer division;

you should shortcut and just use the DATEADD function to add the total seconds:

select dateadd(dd,14249,'19700101') --add the offset for the stored date

--2009-01-05 00:00:00.000

select (1231151827 % 60) --sec to min gain 7 seconds from

select 20519197 % 60 --min to hour gain 37 minutes

select 341986 % 24 --hour to days gain 10 hours

select dateadd(second,1231151827,'19700101')

--2009-01-05 10:37:07.000