To find the next blank cell in column B, place the following eqn in col A:
=IF(COUNTBLANK($B$5:$B5)=1,NOW(),"")
To label every blank row:
=IF(COUNTBLANK(B5:IV5)=255,NOW(),"")
To label the first blank row, if cell A4 is blank:
=IF(AND(COUNT($A$4:$A4)=0,COUNTBLANK(B5:IV5)=255),NOW(),"")
To label the first blank row, if cell A4 contains a number:
=IF(AND(COUNT($A$4:$A4)=1,COUNTBLANK(B5:IV5)=255),NOW(),"")
You can also investigate array formulas to do this type of thing.
Regards,
Brian.