how to auto input date & time, which will not auto update

Dear All,
i want to input current date & time in Cell B1, if i write something on cell A1 then current time will show on B1
i can do this with below code
=IF(A1<>"",NOW(),"")
but time is updating. how can i do that, but time will not change after that,

TIA

1) by iterative calculations - rather not recommended, as this will affect the rest of your workbook
2) by using a Worksheet Change event handler (you need to have macros enabled workbook)

The code could look like that:

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and should be located in the sheet code (right click on sheet tab and select show code from pop-up menu)

See attachment - allow active content and write something in any cell in column A

How about protecting cell Cx when it has been filled?
like:

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But this requires not unlocking them at the beginning

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By the way, as changes are made to next column disabling and then enabling events is not necessary if you limit your "work field" with Intersect

Of course not tested, because somehow you refrain from attaching a file …

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The protection part

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could go into workbook open event (in Thisworkbook module) but as a matter of fact if it has been applied once it does not need to be re-applied every time.
If you decide to do it in workbook open, or keep it in worksheet change handler, I'd recommend rather such approach:

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