Hi, Rick: Here's one way.

Step 1 is to make a backup of your original file so you have a fallback
in case something goes horribly wrong. Assuming your date column is
currently A, insert a new column to the right of that: call it column
B. Enter a formula in column B for each relevant cell in column A:
=IF(A1="",TODAY(),A1)
.... then copy this formula for the remaining cells.

This will use today's date for blank cells, but will show the existing
date if the cell is not blank.

Copy the new column and paste it as values into column A. The original
values will be right where they started, and the blanks will now be
filled with today's date.