Users cannot edit the A/P symbols with CustomMask set to "hh:mm:sst" or "hh:mm:sstt"
RadDateTimePicker, RadTimePicker and RadMaskedEditBox support the custom format 'hh:mm:sst' and users can change AM/PM by using A and P keys with Q2 2016 (2014.2.617) or higher version. In our next release (R1 2017 SP1), we will introduce a way for changing AM/PM with ArrowDown and ArrowUp keys too.