I have just a very light understanding of spreadsheets and want to make one add up engine and airframe hours.

I have managed to get something to add several small number of hours and minutes but there seems to be a 'tipping point' where the answer comes out as nonsense. I certainly can't include the 400+ hours I want to start with, being the figure of the last engine service.

Does anyone have such a spreadsheet with the formulas in that I could have a copy of or can someone tell me what to do please? (I know, pen and paper, but I have numerous mistakes doing that!) TIA