Stopping a count of days when a date is entered

Slight dilema which I am sure is very simple for the majority of XL users to solve. I have a spreadsheet which is populated with 20+ columns, some dates, times and dropdown lists. It also incorporates the Today() function. At the moment when a start date is entered into column H, column G calculates the lapsed time from that column to "todays" date based on the function "NETWORKDAYS". The problem I have is that I want the count to stop for that row when an end date is entered into column L. Hope that I have represented this in an understanderble manner.

Many thanx in advance