Eureka!
The issue was that i dindn't initialized the time correctly.
Yes I followed this post but I used a script that wrapped the /sbin/hwclock --hctosys --utc --noadjfile command, delaying it in time. This let the boot sequence to start already creating a mess with the OnCalendar= entry (as described in the man page).
I was enough to add a service exactly as this, no need to change anything else.