I'm unsure how you determine your billing dates (as in, do they update
automatically? Currently, I can't think of a way that you can manage your
billing dates so that they change given what today's date is, if I am
understanding part of your question..), but anyway

You'll need an IF function as a new column, Column F, (ex. for the entry in
row 4):
=IF(E4<B4,"unearned","paid")
where Column E has your billing dates for each customer, and Column B has
your renewal dates.

At the bottom, or top (wherever you feel like putting it), to calculate the
total unearned money, use the equation:
=SUMIF(F4:F5,"unearned",C4:C5)
Column F would contain the IF condition above and Column C is your Amount
Column.

I hope this helps (there is a way to merge the SumIf with the IF formula
above, but I can't remember it right now, sorry), Jim