Try this
Cell G1 contains the project start date

A2 contains the current date, B2 has the current completion %.
C2 contains =+(100/B2)*(A2-G$2) which will return total project days at current rate
=+G$2+C2 then shows forecast completion date
=+C2-(A2-G$2) will show number of days left befre completion (at current rate)

Hope this helps