If you want to make it as easy as possible on the user, then I think the best option would be to add a custom meta box with three fields for inputting the day/hour/minute values. Using something like a jQuery UI slider style timepicker would further simplify input. Then when the post is saved, use the save_post action to store those three values in a single field.
The second option is similar to the first, but you’d continue using the custom fields as you are now, except a fourth field would be added by you on the save_post action to combine them into a single field you use in your query.