Auto_unique_number

Dear Nader,

I want to create an ID as described in the message below:
Dear Imran,
I myself often use datetime as my id when I need a number format, i.e. string(format-date-time(now(),%Y%m%d%H%M%S%3)). The result will be like 20160506173243556. It includes % of seconds so I do believe it is rather difficult to find the duplicates.
Thien Dao
But I don't know, where to write that on xls form.

Thanks,
Imran