Adding column numbers together

Check attachement if right; if it is, just hide the colored columns (select columns and click on "Hide columns", do not cancel them).

I just extracted day month and year from the last service date (yellow ones); then I told excel to add the month value for servicing (3, 6, 12 but you can also insert 1 or 38) in the orange column; I had also to add something to check if there's a year change (if you are in november aka 11 and you add 3, you won't arrive at month number 11+3=14 because it does not exist; you have to calculate by module12 so it's 14-12=2). If so, the brown column must change and add 1 year... at least! I had to add some other checks and formula to ensure that something that needs revision every 36 months (3 years) shows the right year in the "Next revision" column.