How To Populate Date Cell Based On The Sum Of Two Time Cells

I've attached an example workbook.

Trip Plan example.xlsx

I'm trying to work out how to have cells R11 - Rxx generate the correct date, based on the rest of the calculations.

Please note that the attached is a crude cut down version of the larger spreadsheet, for the purpose of highlighting my problem.

SO, if E7 is changed, subsequently the values in K11:Rxx will be respectively updated.

D12 and G12 are entered manually. (although I'll probably make these drop downs or date choosers at some point...)

Depending on the value in E7 and time in D12, the required break period is listed in K11: Kxx.

The time that these periods of rest are required to occur before is then calculated and listed in O11:Oxx.

I need R11:Rxx to calculated the appropriate date from these calculations, relative to the date in G12.

For example, the rest required in K11, is required on the same day, as such it need to populate with the value in G12.

However the rest required in K13:Kxx, is required by a time (O13:Oxx) on the following day - G12+1

In some case, this time variance could run over 2 or more days.

I figure it's got something to do with the value in D12 also including the date of 0/1/1900.

I'm happy to add hidden cells for formulas/results to work around it, as the end product will print as an A4 document, where many cells will be locked.
Would prefer not go with any VBA unless it's my only option as I haven't done any VBA stuff as yet.

NB (the value in P5 is static only in this example)