How to reference column name pulled from formula

I'm trying to pull a date from columns based on the number in the helper column. I know I can do this with nested IFs, but is there a way to change which column I'm pulling the value from?

My column Start Date should be whatever the column is based on the Index. So for the first column below, since Index is 3, it should pull the value from Day 0 Start3 Date. Instead, it's pulling the text "Day 0 Start3 Date". Can I have the column name be used as a string instead of text?

="[" + INDEX([Index Date Row]:[Index Date Row], MATCH(Index@row, [Index Lookup]:[Index Lookup], 1)) + "]@row"

Thanks!