Excel 2007-Date & Time Issue

Excel 2007-Date & Time Issue
Dear
I am using Excel 2007 with Windows XP. I am facing a problem with the following:
I want to write a daily diary with only two columns “A” and “B” with the titles “Event” in column “A” and “Date & Time” in column “B”. I wish that when i finish my writing the event in column “A” and press the ENTER to go to the next row (not column “B”), the date and time in column “B” must automatically be entered (not changeable, as the event was like a history). And this should go to the whole workbook. What is the method or formula for this issue? Here is a sketch to explain my question more clearly:
EVENT Date & Time
Diana was married today with Raju. The ceremony was very simple but impressive. 26/04/2015 20:37:04
There was a tragic and horrible earthquake in Nepal yesterday, claiming more than 2000 lives. 26/04/2015 20:43:08
Rer...... ?????
r ?
This example is showing that when did i write the event. It must not be changed at all costs, even though i refresh the page.
A bundle of thanks, for those who will solve my problem.
Best Regards, pl reply to:
rafiqgbhsmandrarwp@gmail.com
Rafiq