Usually the work completed field is filled out by the project manager, and not calculated based on time. That would be a time burn rate, not a work completed calculation. If it is 7/1/2007, then you have burned 50% of your time, but you might not have done any work yet. Usually the values submitted are 25%, 50%, 75% and 100%. They are significant of various stages in project completion, so that users don't have to tease out 34.76%, for example.
That being said, do you want all days (364) or work days (260)?
The way you would do this is to calculate the number of days between your start and end dates, then calculate the number of days between your start date and today, then calculate the percentage of burned time like:
I wrote this little script to demonstrate based on total days:
dtStart = #01/01/2008#
dtEnd = #03/01/2008#
dtToday = Date()
iTotal = DateDiff("d", dtStart, dtEnd)
iBurn = DateDiff("d", dtStart, dtToday)
pBurn = iBurn/iTotal
iTotal = 60
iBurn = 28
pBurn = 0.466666666666667
Then display your results with a percent mask.
Does that help?
Look it up at: http://wrox.books24x7.com