I have a similar issue, but in my case the date format doesn't matter because the values I want to freeze are not dates.
I have a worksheet of projected billings spread evenly across months where the value is calculated in each cell. Sometimes the end date is extended, and the remaining fee must be spread over the remaining months without changing the past billings.

Right now i have set a macro that lets users select cells and freeze them. (Simply a Copy + Paste Value)

The dates are at the top of the columns.

But I need to automate this if I can.
Each time the worksheet is opened I need to freeze the cells that occur before the current month. i.e. "<month(today())"

Any time in June, the previous month (M7:Q7 in this case) would be "frozen"

I have attached an image. I can attach the whole sheet if i had to.