Fill Blank Cells in Columns with Specified Header with a Specific Date

Hi everyone

Please see attached.

In the initial sheet, there are columns with blanks (highlighted in yellow). Basically, all I want to do is put some dummy date (say 1/1/1900) in those blanks. This is so that I can run pivot tables with a date filter.

I know exactly the header names, but these aren't fixed and can change columns.

The result is in the Desired Result sheet.

I guess the logic would be something like:
- for each column with a specified header (in this case, Deyt, Date, and Daeet), select blanks until column A's last populated cell row count, and fill these blanks with 1/1/1900.

I'm just not sure how to do this in code.

Any help would be much appreciated.