auto fill-in current date/time and duration

This is my first post on this forum. I used to be an avid user of experts-exchange, but lately all my questions have been excel-related. So since this forum is dedicated to excel - sounds like this is the place to be.

In the following home made time log of activity https://www.dropbox.com/s/0heou3740rpc9os/Tasks.xlsx, I would like the following to happen.
When I enter the current task into A3, I want C2 (i.e. end time of previous task) and B3 (begin time of current task) to become auto-populated with current date/time and I want D2 to become populated with duration (of that previous task, of course).

How would I do that?

Thanks so much!
Alex

i'll try to answer... im just a newbie myself but i hope i can help to get you started..

in B3, write this =IF(A3<>"",C2," ") (this will copy whatever is the end time of previous task as soon as a task name is written in column A)
in C3, write =NOW() then just copy paste special the values
in D3, write C3-B3 (make sure that column D is formatted as hh:mm:ss)

just drag down the formula to the last row you need

hope this helps somehow.... otherwise let's wait for the experts.

Thanks for such prompt response, but I want this to happen automatically without having to fill anything in each time. All I want to do is enter the text into the task field and hit enter.