Good evening

I have the following question

When there is a value in cell A1, the date and time has to come auto in cell B1. cell B has to be locked (with protect sheet) and the date and time may not change so has to be fix.

example:
cell A1: 123456
cell B1: 05/09/2014 17:19


what i have so far is a formule =IF(A1;NOW();"") in locked cell B1. But the date and hour change. this is logic because i use the function NOW.

Is there someone who can help we with this issue.

I hope that it is a little bit clear??

many thanks