What should We Write First

Now we have our simple pseudocode:

days = 0
while date1 is before date2:
    date1 = day after date1
    days += 1
return days

What should we write first?

daysBetweenDates to solve the whole problem

nextDay(year,month,day) to get next day for simple case

isLeapYear(year) to determine if year is leap year

daysInMonth(month) to get the number of days in a month