A time calculator I made for personal use. You'll also need the NUM2STR program.
Output is stored in Str0.

So, originally, I wanted to convert thousands of seconds to minutes and hours, then decided I needed to be able to add/subtract those converted numbers.

So you just input the H:M:S numbers at any positive value at the prompts, you can even add or subtract in these prompts, and it outputs the total H:M:S.