Implementation of ACT/365A.
Calculates the day count by calculating the actual number of days between startDate and endDate.
A fractional day is rounded to the approximately nearest day.
The day count fraction is calculated using ACT/365A convention, that is, the
day count is divided by 366 if February 29 lies in between startDate (excluding) and endDate (including),
otherwise it the day count is divided by 365.