Hi H_a_l,
It seems that I am getting the same issue when using the similar function with you.
Though without using the function Filter( Library, Created=Modified), I can get both the timestamp for Created and Modified.
While when using the function, it returns an error saying that “value doesn't fall within expected ranges”.
Then I use the following function, and it works for me:
Filter('Test library1',If(!IsBlank(Modified),Created=Modified))
Please have a try with it.
Best regards,
Mabel Mao