Now we have Xposed Framework (As of Oct 10, 2017) and GravityBox [N] (As of Oct 11, 2017) for Android N!
As a regular notice, flashing Xposed Framework may cause instability and unexpected behavior of your phone. Proceed at your own risk.
Get both of them, flash the Xp Framework via recovery, then install GB and reboot. Now open GB and navigate to Status Bar -> Time Modification. There you'll find the option named "Show date in status bar". Tap on it and it'll immediately take effect
Problem solved :)
P.S. I've never heard that Android has this function rawly supported. Is it a custom ROM?