Thanks so much in advance for your help. I'm very new to this stuff,
so I probably won't explain myself very well. I'm thinking a macro is
what I need, but I'm not sure.

A3 is the current date.
L9 is the current number of mutual fund units that a client owns
G9 is the amount of money the client invests into the fund every month
on the 5th.
M9 is the current price of the mutual fund.

I need a trigger that will update L9 on the 5th of each month giving
me the new unit total based on the fact that the amount in G9 has now
been invested into the fund.