I don't believe the BS5 can start a timer. It can only respond to timers from a video master. At least this is what the FAQ on the B&O site implies (no mention of starting a timer). Also, the option to select "play timer" from the roles menu is only available when the BS5 is configured as an "audio master" which means there is no other audio master in the system - hence, only a video master can do this:
Can BeoSound 5 be part of a Play Timer?
Yes, the speakers connected to BeoSound 5 can be set to start up when a Play Timer programmed on e.g. a connected video system, is executed.
To make BeoSound 5 part of a Play Timer…
> Select SETTINGS in the MODE menu
> Select PLAY TIMER in the ROLES menu
> Move the wheel to select ON and press GO
Here's another possibly useful FAQ entry:
Why does a Timer Play with a BeoSound 5 'N.Music' or 'N.Radio' source not start up?
When you program a Timer Play on your Bang & Olufsen video or audio product you can select either 'N.Music' or N.Radio from BeoSound 5 as source.
When you select N.Music the content from the red 'Favourites' list on
BeoSound 5 is always selected automatically. This means that if your red
'FAVOURITES' list is empty, Timer Play
will not start up as programmed. Therefore you always need to add the
content you want to listen to, in the red ‘FAVOURITES’ list.
When you select N.Radio, you select a ‘Channel’ and the content from the
numbered preset list is automatically selected. If the Preset list is
empty, the system starts automatically on the last played radio
station.
Stan