Jack Sheet wrote:
200000 is invested in an account that pays 5.05% APR (fixed). Interest is
compounded daily.
I am trying to get an Excel formula that returns the amount of interest for
a specified number of days.

If by "APR", you mean APY -- that is, the compounded rate, not the
nominal rate -- then the daily rate can be computed by:

=rate(365, 0, -1, 1+5.05%)

Alternatively:

=(1+5.05%) ^ (1/365) - 1

In either case, you might need to change the cell format to Percentage
with 4 or more decimal places (Format Cells Number).

Thus, the amount of interest accrued over 13 days can be computed by:

=fv(A1, 13, 0, -200000) - 200000

where A1 is the daily rate. You might need to change the cell format
to Number or Currency with 2 decimal places.

PS: If by "APR", you actually mean the nominal annual rate, then the
daily rate is simply 5.05%/365.