I have the same problem, putting my S5 into silent mode via the new interruptions system is a pain in the ***, the options that those with a stock android phone are not there to do it easily and the scheduled silent mode isn't there at all. Its pretty pathetic TBH.......
drag down to get to notification panel
tap and hold the sound button until the options menu appears
scroll down until interruptions can be seen
tap interruptions
tap ' on receiving call and notifications '
select ' do not interrupt '
press home button.
All that just to silence the phone. If there's another way please let me know.