Is the column a numeric or a character column? The proper way to have a numberic column, and to specify a format that will let one display the data in one of several different date formats.
If your data are character and you just want to add in the semicolon, the script below will do that.
Assuming your column is called "Date" here is the script
Names Default to Here( 1 );
dt = current data table();
For Each Row(
:Date = Left( :Date, 2 ) || ":" || Right( :Date, 2 );
);