Returning nearest date based on dynamic data

I need to be able to return a date for the previous and next dates based upon the current date and interval between dates.

I have attached a model as rough idea of what I need.

Essentially I have the "=today()" formula in a fixed cell to make life a little easier. I have a variable titled Interval which allows the data enterer to select an interval from a controlled list. Then I have the Last and Next columns which need to show the date of the last issued invoice and the date that the next one will be due.

The first pay column is a fixed date that will not change.

The day of each month will always be the same as the day of the first payment (e.g. 5th will always be the 5th regardless of month, And if its weekly it will always be on the same day), at this stage I'm not worried if it falls on a weekend for anything other than weekly.Attachment 438481

Cell F3: Last Inv

Formula:

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Cell F4: Next Inv

Formula:

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Note: This only covers weekly and monthly.

Thank you for the help, that is a good starting point for me (there are a few other bits I need to do with it which I have not posted here).

Hi
try these for Last Inv in F3:

Formula:

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