from datetime import datetime

input_str = "200099"

Manually split the string

full_year = input_str[:4] # First 4 characters represent the full year
short_year = input_str[4:] # Last 2 characters represent the short year

Parse separately

date_full = datetime.strptime(full_year, "%Y")
date_short = datetime.strptime(short_year, "%y")

Now, handle both years according to your requirements

print(date_full) # Output: 2000-01-01 00:00:00
print(date_short) # Output: 1999-01-01 00:00:00

Here I'm just separately parsing both parts of the year and then I guess you apply them as needed?