Hi, I need to calculate the difference between children's actual age and
their tested reading age.

I have managed to set up a spreadsheet that will work out their age based on
Date of Birth, expressed as custom yy"y" mm"m" e.g. 07y 08m

I now need to be able to type in their tested reading age (e.g. 08y 03m) in
the same format, how do I do this?

And then I need to calculate the difference, (rounded to the nearest 6
months).

Please can someone help me! All the teachers I know will be eternally
grateful!