940 rule
The 940 only needs to be added if the sum of the minutes digits is more than 60.
It works out fine for even two or greater digit hours..
14:58 + 12:56
14058 + 12056 = 26114 + 940 = 27054 = 27:54
14:15 + 15:13 = 14015 + 15013 = 29028 = 29:28 (no need to add 940)
However the limitation is that only two figures can be added one at a time. More than that you need to do it in multiple steps.
But, really cool otherwise