apply inflation at annivsery period to prior value

Hi Forum,

Hopefully you can help.
I'm trying to work out how to create a forumla that applies inflation based on a a dynamic anniversary period.
for example I have an amount in month 1 of 10,000 and the annivsary is in 36 months time, i want the forumla to look at the date in the row, check if its 36 months from the start date and then should add say 5% inflation - so 10,500, then again in 36 months time it needs to add another 5%, but to the previous amount of 10,500.

attached might help to explain better.. this is driving me mad.

attached with expected result, I had a go with a forumla I found on previous posts and it kind of works at the anniversary date,but can't work out why its produces error values.

superb Glenn, thanks a million! much appraciated.