Hi Karthik,
When you calculate the days in between it is logical that you get a number back. This number is the amount of days that are in between the two dates.
If you want to add or substract a number of days from a date, you can use the function addDays.
example to add:
addDays( [%BeginOfCurrentDay%], 1 )
example to substract:
addDays( [%BeginOfCurrentDay%], -1 )