If you have a Work Order for a specific day with a specific tech, you don't have to assign a specific time to it.

Create a Work Pool.

Or - 

Create the Work Order and assign it an 'off hours' time, such as 3 AM. The tech sees it on the Mobile app, but knows the appointment time is not important.

Note Let all your techs know that when they see an appointment at this hour, it means they can go anytime.