excel - Adding months together or working in base 12 -

i'm teaching assistant school working kids special needs , i've been asked produce spreadsheet can keep track of reading ages , spelling ages progress through school. have 2 tests per year, every 6 months, i'm using column test result , adjacent column improvement on last test.

however, in order improvement work properly, need cells h2 (for example) v20 work in base 12 can perform functions on it. i've found way of converting output of call display in base 12, that's can use - base 12.

an example of how i've got follows:

reading age after year 7 test 1 - 7 reading age after year 7 test 2 - 7.2 (not 7.2 years, 7 years , 2months.) reading age after year 8 test 1 - 7.8 (again, 7 years , 8 months) reading age after year 8 test 2 - 7.11 (so rather being 7 units, 1 tenth , 1 hundredth, 7 years , 11 months)

the formula work out improvement on per test follows:

=if(or(isblank(h2),isblank(i2))," ",(i2-h2))

my issue getting cells within area work (or think?) if working months, rather decimals.

i'd suggest need use 7.01 7 years 1 month can distinguish between , 7.10 (7 years 10 months). if use approach can use dollarde , dollarfr functions handle base 12, e.g.

=if(count(h2,i2)=2,dollarfr(dollarde(i2,12)-dollarde(h2,12),12),"")