Exponentially decaying rank accourding to datetime in c#

I am looking for helpon how to give an exponentially decaying weight according to time. I am writing a small web searching program in c# and want to rank search results according to a time decaying factor. Can anybody help, i am a newbie 开发者_Go百科to ranking algorithms.

1) Subtract your time values from your reference time (in a real time environment that would be DateTime.Now) to give you a Timespan. 2) Using this Timespan, use one of the Total methods (i.e. TotalHours) to get a double value. 3) If you want to do exponentially decaying weights, then try looking at this:

Exponential Moving Averages

Exponential is called exponential because of the properties of a mathematical exponent. Take the age of the result and do the following:

rankAdjustment = age ^ age;

That will give you an adjustment to your rank that exponentially changes with length of time.