To add/remove date from the date string you need to use AddDay() method. For example,
We need to add 6 days to the current date.
(Get-Date).AddDays(6) 25 March 2020 22:40:36
To go back to the specific number of days, the same AddDays() method will be used but the numbers are negative.
(Get-Date).AddDays(-6) 13 March 2020 22:45:34