Reckoning is an Attitude as well as an Action.
An action is taken once, but an attitude is constantly maintained.
An action is taken as a timely move against a certain matter; an attitude is maintained as a lasting estimation of that matter.
We ought to Reckon ourselves as dead to sin.
This is to say that we should take a singular action of Reckoning ourselves as dead, and then follow through by a permanent attitude of Reckoning ourselves as indeed dead to sin.