One of the reasons I learned to program was to be able to create tools for myself. When I do so, I'll put them in this repo. Mostly it will be little snippets of HTML5 with CSS and JS.
Time Addition Calculator
The idea behind this is that you're looking at an online course syllabus with 9 or 10 videos in it. You want to know how long the whole thing is, but the time estimate (if there is one) includes time for exercises, projects, etc. You just want to add up the times of the videos.
With the Time Addition Calculator, you enter times with dot separators so it's easy to do quickly on your numerical keypad. You enter hours[optional].minutes[optional].seconds. Hitting the plus (+) or enter keys on the keypad finishes an entry (if it's just numbers and dots - if not, nope). You can retroactively remove the last entry from the list or clear the whole thing.