I have the same problem. I am trying to add the following durations:
A + B = C
1:12 + 2:24 = 3:36 (in English, 1hr 12min + 2hr 24min). I'm trying to convert the PHP program I created to a Numbers spreadsheet for offline ease of use.
If A and B are formatted as durations, the only way to enter properly is to type HH:MM:SS, which is quite painful when entering thousands of entries, especially when I have no need for seconds (this is already painful, as I actually write durations in this format: HH.MM). The C is accurate sum of the durations, though it is a bunch of unwanted typing.
If A and B are formatted as times, however, I can enter 1:12 and it formats properly. However, SUM does not work for multiple time values. So it appears I get the formatting I want when using date/time, though I can only perform the calculation when using duration.
Regardless of what I select in the format drop down box (I have HH:MM selected), this requires the input of :SS in order to format properly (otherwise typing in 1:12 reformats to 0:01, which is not as intended).
Thanks! It appears that Numbers' expectation to also use seconds in the duration cells is what's causing my (and OP's) heartaches. As a workaround, maybe I could just type 1.12+2.24=3.36 (as I have in my PHP program, which treats decimal and colon as the same thing). I don't know how to "explode" the integer and decimal portions of the number though. Any ideas? Thanks!