Since 20=24-4, adding on 20 hours is the same as subtracting 4 hours. For example, if the current time is 11.15 then 20 hours later it will be 7.15 (the next day).
If the current time is between midnight and 4am, then just add 8 hours (that's 20-12) and change am to pm. For example,
3.25am plus 20 hours is 11.25pm.