Modify a date

[ValeriaBonini]

I have a date field with many dates (format DD/MM/YYYY), i would like to create a new column with these conditions:

if date is between 13/08/YYYY and 17/08/YYYY -> date become 15/09/YYYY

if date is between 13/12/YYYY and 17/12/YYYY -> date become 15/01/YYYY

how can I do this?

[vinieme12]

@ValeriaBonini  have you tried the above? did it work? 

[ValeriaBonini]

I tried, but it doesnt shows the right format