Formulas for adding durations

I have a column of durations. At the bottom, there is a monthly total, and underneath that is a cumulative annual total. I only want these two totals to show IF I have invested any time... because I don't want my efforts to get lost in an ocean of zeros, or 0:00's, and I just simply want a cleaner spreadsheet. So I created a custom cell data format that I call "Time Duration or Blank". If there is no time to add, this data format will will make these two totals appear blank, even though it is really a 0 in the cell.

I have twelve spreadsheets - one for each month. So far, the totals for the month work great. But I cannot seem to find a formula (I am fairly new to cell spreadsheet formulas) which will add the cumulative totals from month to month. It DOES work if BOTH the cumulative total from the month before AND the current monthly total contain a duration. But if either or both does not, I cannot seem to come up with a formula to handle that.

Thanks for any help.