Using "current_date()" in custom field

[kerriu]

I would like to count the number of days that elapse between an event and the current date. When I use the below syntax in the custom field, I get this error: Unknown function "current_date"

diff_days(current_date(), ${timeclock_employee_hours.time_in_date})

Any idea how to get a custom field calculation to return an integer for difference between current data and timeclock event?