In the computation of time necessary for prescription the following rules shall be observed:
(1) The present possessor may complete the period necessary for prescription by tacking his possession to that of his grantor or predecessor in interest;
(2) It is presumed that the present possessor who was also the possessor at a previous time, has continued to be in possession during the intervening time, unless there is proof to the contrary;
(3) The first day shall be excluded and the last day included. (1960a)