Help a poor SEN teacher out! Year:Month calculations :/

[heisenburg]

I'm putting together what should be a very simple tracking sheet for monitoring progress of my pupils who are undergoing litercy interventions.

I want to be able to input a baseline data reading age and then at the end of the intervention a re-assessed reading age. I would like the tracker to automatically calculate the difference (progress a pupil has made).

Reading ages are measured in Years:Months (using the colon). For example 06:09 would be Six years and nine months.

Colons are used as time in excel and that's where I'm hitting problems - I don't know how to get around this. If I have a start RA of 10:03 and a second age of 11:04 then I get progress of 01:01 which would represent an increase of 1 year and 1 month. However, if I input a start age of 10:03 and then a second age of 0:58. The calculation currently used is a very simple =sum(A2-A1) i.e 11:04 - 10:03 = 01.01

Ok, I'm sure there's a very simple solution to my problem but I just can't put my finger on it! If you have a solution or a suggestion I'd be super grateful!!!

I've attached what I have in the hope that some super-excel guru can help!

Thank you