[Javascript] Survey and elapsed time behind the scenes

[dev at qroute.net]

I am running a survey. There is a reward at the end.
So, those who take their *time* to take the survey are taken into a
sweepstakes which may entitled them to get some prizes.

In the survey, there are many required fields. So people are forced to take
the time to read it.

To filter out the incoming submissions from the junky ones, I plan to deploy
the following technique.

When the page that has the survey form loads, I plan to start a javascript
timer behind the scenes and to measure time it takes for the user to respond
all the questions. Of course, no one is going to know about it. ;-)

At the end, right at onSubmit, a hidden field will be populated with the
elapaed time info at that time so the elapsed time as well can be shipped
from the client to the server.

That elapsed time will enable me to eliminate those survey's that took say
less than 60 seconds to generate.

I'd like to hear your opinions on this idea. Pro's and con's and new ideas.

But I also need you to help me with coming up with that elapsed time
implementation. A reliable function is what I need.
Thank you