Which hook to alter field content before it is formatted?

I'm trying to write a basic custom module to alter field content before it is formatted, but I'm not sure which hook I need to implement.

My scenario: I have a Regular Hours and an Hours Exception content type each with an office_hours field. Based on a date argument, I want to alter the Regular Hours office_hours field before it is formatted with the Hours Exception information, if it applies.

It seems like this must be possible, but I can't figure out which hook to implement. I'm not even sure whether the hook should be a node hook or a field hook. I've been looking at hook_node_load primarily, but without success. Am I on the right track?

Thanks for any light you can shed on this.