Adding Days to a date without using a library

Question

I want to add a number of days to a given date without using any library I did this part of code :

   Days_in_Month = [31, 27, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

if isBisextile(year):
   Days_in_Month[1] = 28

days = int(input("Enter number of days:\n"))

while days > 0:

   if isBisextile(year):
       Days_in_Month[1] = 28
   date += 1

   if date > int(Days_in_Month[month-1]):
       month += 1
       if month > 12:
           year += 1
           month = 1
       date = 1
   days -= 1


print("{}/{}/{}".format(date, month, year))

If I test with, for example:

Enter a date:
2/7/1980
Enter number of days:
1460

It yields 2/7/1984 instead of 1/7/1984

Does someone have idea why I have plus one day ?

Creating a library like this is a good way to learn but, and I can't stress this enough, should not be done for code you are relying on. Assuming this is a given but want to put it in for posterity. I have seen lots of bugs from people trying to recreate datetime libraries. It's typically error prone and every language has datetime functions to rely on. — smcjones
– smcjones, Commented Nov 24, 2019 at 23:58

David Buck · Accepted Answer · 2019-11-24 23:52:39Z

This appears to have fixed your issue:

Days_in_Month[1] should be 28 or 29, not 27 or 28 and it needed to be corrected both ways for each year.

I wrote my own isBisextile() which is obviously not fully compliant with all of the rules of leap-years that are % 100 or % 400 but I assume you have that covered in your version that you haven't shown us.

year = 1980
month = 7
date = 2

Days_in_Month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
def isBisextile(year):
    return True if year%4 == 0 else False

if isBisextile(year):
   Days_in_Month[1] = 29

days = int(input("Enter number of days:\n"))

while days > 0:

   if isBisextile(year):
       Days_in_Month[1] = 29
   else:
       Days_in_Month[1] = 28

   date += 1

   if date > int(Days_in_Month[month-1]):
       month += 1
       if month > 12:
           year += 1
           month = 1
       date = 1
   days -= 1


print("{}/{}/{}".format(date, month, year))

For 2/7/1980 and number of days: 1460 it gives 1/7/1984 as expected.

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