So the MonthToDate function shows the range of the first day of the month to 'today'.

However I need to calculate the number, in days of that range to use in another calculation.

I tried to create a custom formula to calculate the difference between the first day of any month and current date, but I get an error saying that I need to enter in an actual date:
DateDiff ("D",Day (1) ,CurrentDate )
"A date is required here"

I was also thinking if there was a way to convert a range to a numerical value but havent been successful.

What do you all recommend the best way to approach this?

day(currentdate) gives you the day of the month wich I think is the value you need.