I have 2 fields 
Field 1: Date (lookup)
Field 2: I wanna if the date (and time) Duplicate , just show 1 value date time . If it has 2,3 different date, i wanna show all of them
And i use this formula and it shows error, what am i missing ?

Arrrayunique for Date (lookup field)
Best answer by ScottWorld
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.
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