Welcome to the community, @Kendy511!

Your formula is trying to format an entire string of multiple dates, as opposed to an individual date.

However, even if you moved the DATETIME_FORMAT inside the parenthesis, it still wouldn’t work, because one of the huge limitations of working with arrays in Airtable is that the array functions are so limited in their functionality that you can’t add anything else to those array functions. You can only use them as-is.

So your formula must stay as: ARRAYUNIQUE(values)

However, what you can do is go back to your OTHER TABLE and create a formula field there that formats the date the way you want there.

Then, in this table, change your Rollup field to point to the formatted date field in that other table.

Well, i’m appreciate your suggest, I tried to go back to OTHER TABLE with single Date time like this

And create a new column nearby to convert Date time to value. So what exactly formula I can use to change Date time format to values but still keep the display Date & time as the old one ? @ScottWorld

In your other table, create a formula field that looks like this:

DATETIME_FORMAT({Date Field},'DD-MM-YYYY')

(Replace the words “Date Field” with the actual name of your date field.)

Then, back in your original table, create a rollup field that rolls up this new formula field.

well, That seems an normal work but need a lit step to convert . Thank you for your helps!