General Caster with Mirror columns

Hi there.
I’m trying to use General Caster. I have a high level board with only mirrored columns which I want to export to Excel. As you know it doesn’t work properly (exporting all values in date columns as an example).

I’m trying to use General caster to show me some mirror columns and then to export it properly to excel, I’m trying to use the 'Whenever there is a change, use formula (and I want to show one of my mirrored date field), and placed it in another column (date field).

When I do it, the result is BLANK… what should I write in the ‘Formula’ please?
Thanks so much,

Please contact us at reporting the account name you are using GC on.

@DavidR I am having this same problem. The mirrored column doesn’t show up as selectable in the list of columns. Did you figure out what was going wrong?

I gave up on using general caster.