Formula Fields lack decent capability

Formula Fields lack decent capability

I am looking to create a field that shows the `Number of days since` a particular date. The date in question is the `Last Invoice` date, which is added to an account record via a deluge script.

I currently have a script that updates this `Number of Days since last invoice` number, but unfortunately every time the script runs it also changes the modified date as well, when it is not really an update and also uses a lot of API.

It would be great if I could use a Formula Field, but there doesn't seem to be the correct functions available to do this.
Now() - ${Accounts.Last Invoice}

Datecomp() seems like it should do the trick, but it returns minutes.

It would be great if the available functions were improved and new functions made available.