Hiring Services - PO & Service Entry Sheet

Dear Team,

Client has given service order to vendor for Hiring of Crane for 12 months. So in PO quantity 12 & UoM is MON are entered.

But vendor perform services on Hours or Days basis. Means he can send a bill for 26 days or can be in Hours also

So how to create Serivce Entry Sheet in Hours or Days while PO is in MON [Month basis]

Pls suggest

Thanks