How can you subtract a date field from the current day's date?

if you have a formula numeric field, the formula would be

ToDays(Today()-[date field])


The today() function returns today's date. The similarly (but different) named function "ToDays ( )" converts a duration result to the number of days, i.e. a number.

Tried to use this but it says the minus sign does that work for date or datetime field types.

if the date field is a date/time type, you may have to have something like:

ToDays(Today() - ToDate([Date/Time Field]))

You could set up an "If" statement if you have a "Project Start Date" but I think the field would have to be a formula text field:

If(
not IsNull([Project Start Date]),"N/A",
ToText(ToDays(Today() - ToDate([Assigned Date]))
)
)

unfortunately with this is if you sort by this field, it will sort alphabetically and not numerically. For example, if you had the data: 1, 2, 3, 4, 12, 22, 42, N/A
it would sort to this:
1
12
2
22
3
4
42
N/A