I have a Tenant who was on a 6 Month Assured Short Hold Tenancy which rolled on to a monthly statutory one after the first 6 months. The Tenancy commenced on 16th January 2010. They gave me written notice to terminate the tenancy on the 20th February 2017 and moved out of the property on 22nd March 2017. Are they liable to pay the monthly rent in full, that was due on 16th March 2017? Hence their leaving date would legally be 16th April 2017.

What were the dates (start, finish) or original tenancy & was rent payable monthly? It's not rent day tenant notice should expire on but end-of-period day.

However, sounds like you've already accepted their notice, it's a bit late to start arguing: In future, if you wish to dispute, you'd need to formally reject notice &, to be gentlemanly, point out the error & request a valid notice.

Further however, you have had tenants who actually bothered to give notice & then left, without the need for you to issue notices, take them to court, expense, arguments, missed rent etc etc... Me, I'd accept quite a bit of lee-way & be happy.