Atlus has updated the official Japanese website for Persona 5, with a link that takes you to a countdown page. At the moment the countdown is at 320 hours. We do not currently know what the countdown is for, however it is set to end at 9:30pm BST on Thursday 5th May 2016. That is Friday 6th May 2016 at 5:30am JST, and Thursday 5th May 2016 at 3:30pm EST.
If you are interested in seeing the countdown for yourself, then head on over here.