Well, the picture explains a lot.
First of all, your numbers of seconds are much more than something like a few hours (or a part of it, as in your original example), so it doesn't make sense to add it to a time without date, unless you acually want to start at December 30, 1899, 7:00 AM.
(Date numbering in the Power.... environment is based on the numbering in Excel starting with 1 on 1/1/1900, but unfortunately the year 1900 was considered a leap year.... so day 60 in Excel is the non existing 2/29/1900; day 61 is 3/1/1900 both in Excel and the Power ... environments, so calculating back to 0 brings you to December 30, 1899, 12:00:00 AM (midnight)).
If you are adding a number to a datetime as a new column, then you are adding days.
So if you want to add seconds, you need to divide the number by the number of seconds in a day, being 86400 (24 * 60 * 60).
Your example of 1,555,200 seconds brings you to 1/17/1900 7:00:00 AM.
Watch this video, to which I added a short demo of Camtasia Studio, the software I'm using to create videos.
