excel - Update current time and date if a column value changed to predefined text -

i want column add current time if specific column have predefined text.

in cell of b column have written formula update time in basis of column text.

=if(a2="resolved",=now(),"not done") 

this shows error.

i want apply formula entire column. tied:

=if(a2="resolved",now(),"not done") 

but time in column b gets updated same value if cell changed in column.

this because have worksheet set auto calculate. time condition true, or cells in column match condition, cause current time stamp used.

the way prevent changing cells matching condition change manual calculation, have select cell , press enter perform calculation. unfortunately disable automatic calculation entire workbook, other formulas not update either. unfortunately there no way on cell cell basis, other use vba, trigger if cells changed.