Not exactly sure what you are wanting to do... but you can do it like this.
Replace col B with following.
=IF(CEILING(A1,1/48)=CEILING(MOD(NOW(),1),1/48),1,0)
Or alternately.
=--(CEILING(A1,1/48)=CEILING(MOD(NOW(),1),1/48))
Note that you need to use MOD(,1) to extract time portion only (which is expressed by values after decimal point in Excel).
See attached.