Ifelse() usage with calculated fields?

I have Start Date a field in dataset and Startdate is a calculated field created by parameter, i would like to create a calculated field by using both of them like below
ifelse(
{Start Date}<{Startdate}
,{No.of Rental days for the current month}
,{Actual No.of Rental days}
)
but it is throwing error.

I use calculated fields in functions all the time.
My guess is that: {Start Date}<{Startdate} is not a valid expression. I’d try:

ifelse(
dateDiff ({Start Date},{Startdate}) > 0,
,{No.of Rental days for the current month}
,{Actual No.of Rental days}
)

If that does not work, what error are you getting?