Calculation of period of holding

FOR THE PURPOSE OF CALCULATION OF CAPITAL GAIN THE PERIOD OF HOLDING SHOULD BE TAKEN FROM DATE OF REGISTRATION or DATE OF POSSESION ?

iF INTHE CASE A PROPERTY IS REGISTERED AND THE DATE OF POSSEION IS LATER THEN CAN THE ASSESSE CLAIM EXEMPTION OF SEC 54 IF HE REINVEST IT IN NEW HOUSE PROPERTY?

PLS ADVICE WITH CASE REFERENCE IF ANY