Gantt Chart Automate

I just download a free excel Gantt chart template (see attached file). The goal is to calculate the percentage complete automatically for the sub tasks. My thought process was to determine how many days from present date (see cell C8) and the start date of the subtask (see D14) and subtract that from the days remaining in the subtask (see cell H14). Then convert into a percentage (see cell G14).

The problem I have is when the subtask has not stated as of the present date the output is a negative number.

What I want to happen is when the output or sum is negative than it should be zero. That way the percentage will be zero and the Gantt bar will not move. Please let me know if more information is needed.

Any help or hints will be greatly appreciated.