@Nidutt you should be able to use NifI expression language in the flow to change date int to ISO timestamps.
Here is a template you can use that shows many examples of timestamp formatting:
https://github.com/steven-matison/NiFi-Templates/blob/master/Working_with_TimeStamps.xml
I think you may find that nifi attributes remain strings in your flow without a strict date type, after all an ISO timestamp is really a string, your end point database just knows it is a "timestamp"....
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Thanks,
Steven @ DFHZ