What's the Date after N Days?

Create a function that returns the date in DDMMYYYY format after a specific date. Consider leap years and leap months (e.g. February 29th). Please do not import anything (such as datetime).

Examples

add_n_days_to_a_date("15041984", 6038) ➞ "26102000"

add_n_days_to_a_date("26102000", 6038)) ➞ "08052017"

add_n_days_to_a_date("01011900", 30) ➞ "31011900"

Notes

• Remember that if the year is a new century like 1800, although it is divisible by 4, it isn't divisible by 400, hence it's NOT a leap year.
• Please give feedback if there are any bugs (this is my first time making a challenge).