Trying to experiment with IPFIX instead of NetFlow, my monitoring software reported flows from 1504215292 seconds ago and promptly discarded them.
First I thought it was some misconfiguration but after trying to change back to NetFlow and seeing everything working fine, it ocurred to me...
As it turns out, 1504215292 seconds equals to exactly 47 years, 8 months and a few days, with some quick calculations this brings you back to 1970 which is also the default date in RouterOS before any date configuration/synchronization.

RouterOS version is 6.38.7 (bugfix)

As I understand 1504215292 is Unix Time Stamp. That equivalent 08/31/2017 @ 9:34pm in UTC time
You can convert Unix time on this website.