Hello Carranea34
Welcome to the SBB Community.
It is for 24 hours, not per calendar day.
You cannot count on their availability as Interlaken Ost is a very busy station and you are likeky not the only one to use this service. Alternatively, you can store it at the staffed baggage office during their opening times or change plans.
Interlaken Ost station is run by BLS, not SBB.
Kind regards,
Roland