I am not sure why you would need to do this as it is a long way off and the timing would become clearer then.
You could perhaps ask a clan member to gpo you, but I'm am guessing it might just give you the same answer of "3w 1d" .
IF you really need to know exactly just make a note when next applying nearer its expiration.
Make the peace treaty which you use most often a "favourite" and that allows you to see the number of days and hours remaining on your shield by clicking on "Boosts".
I commend the practice of maintaining a shield of a few days or a week at all times. The game is generous (currently at any rate) with 8 hour peace treaty items and you can use many of these every day without running any risk of running out. Letting a shield run down to just minutes left is an unnecessary risk.
Click on Boost icon (above Events icon which is the one with the sword on right hand side of screen) then click on Military tab where you will find Peace Shield and that will show if you have an active shield and for how much longer.
As others have said, if you make at least one of your shields a favorite then it will appear when you hit the Boost Button or "B" if playing on web not mobile. Good luck.
Just realised, you want the exact calendar day and time when it expires; apologies. I don't worry as if it says 3 weeks 2days left and I'm away for a week I know I'm covered. If in doubt I add more.
If you want exact time etc then get a friend to Guard Payoff you and let you know the result.