Glad you found the solution, but i really believe it's wrong...
For 2 reasons: you create a circular reference by refering in the formula to the cell where the formula is placed, and second reason is that if you eliminate the first problem, and you have a value in A1, your formula will put a formula in B1: Now(), formula that will permanently refresh, and you said you need the time when data is entered in A1, not the Now() value..
To insert a permanent date and time value in a cell, can be done with a macro, targeted to A column