How do I add days?

I make an app when entering child's date of birth, a list appears for him or calendar of the upcoming vaccination dates, for example the user enter 12/12/2020
calender or list appear had dates of the next vaccinations,
the idea is where I compare the date that the user entered and add certain days to it , for example, user enter 12/12/2020 then calender or list appear upcoming vaccinations will be in 18/12/2020
The problem I have is that I cannot do the correct comparison and add days

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