Date & Time stamp for change in value

HI

Can Any body help me with VBA or any excel formula, by which a chaneg in value of a a cell to be written with the Time & date at other row.

Ex: Cell A1 relates to the changing value of stock. At perticular point it should write BUY
in cell B2 it should write the time& date at which the Value of A is Buy or Sell If any change in value once again it has to write it in next row. Not in the same row

Any Experts pls help me........