"IS that an easy thing or should I be scared of attempting it?"
That largely depends on you. How much experience do you have working with relational databases in general and FileMaker Pro is particular?
It can certainly be done, but requires understanding how your database file currently works before you can make the necessary changes to it.
Ok, erm, to be honest the question was rhetorical.
I have absolutely NO experience of working with relational databases and Filemaker Pro. That's why I'm adjusting a template rather than starting a database from scratch.
I can see that in the "process Check Out" script, the issue is the lines:
Set Field [History:: Check Out Date; Get(Current Date)]
Set Field [History:: Check Out Time; Get(Current Time)]
I obviously can see these need to be adjusted to give you option of entering a date and time in the future, which I assume also means you'd need to create another field and place a space for it in the check out window.
What I was sort of hoping was that some whiz would come along and tell me what adjustments I need to make to the script to make it do what I want!
Your analysis, as far as it goes, appears valid. You could certainly define a date field and then replace Get ( Current Date ) with a reference to that date field. The same is true for the time field.
But without taking a great deal of time to analyze the structure and function of your database, I can't really tell whether this may keep other functions of your system from working correctly.
I good book on FileMaker and/or watching some tutorials on FileMaker would help you get off to a better start with this task.