[Dach]

The only way to find the exact date at any time you need to iterate from the beginning date and acrue your date that way.
There isn't any real way to derive the exact date from a bunch of seconds with some simple equation. There will always be some days that your year will not come out right.

Note: the way you an I both derive the year in our scripts will produce bad results when nearing the turn of the year on certain years. I know mine will be off at the end quarter of Dec 31 if there is but one leap year between the current date and my subtracted date. I don't really feel like figuring out exactly when yours will be off, but I'll guess that it will be on the first portions of Jan 1 on all non leap years.