I'm not a numbers guru, but a quick Google sent me to this page, which explains how to set the format for a particular cell or range of cells:
Format dates, currency and more in Numbers on Mac – Apple Support (UK)
The last two options three (Date & Time and Durations) are the ones you need. The tricky thing is setting the format in such a way that it sticks to the cell(s). I would have a go at changing the cell format for the outcome of your equations to show a duration and see if that produces a correct output. If not, you may need to delve into the vagaries of date and time calculations in Numbers. I suspect that, just like in Excel, there is a simple number in the background that gets translated to a date/time based on the cell format. I do hope they use the same number format with whole numbers for days and fractions for the time element.