Question

Is instantaneous velocity calculated as displacement/specific time?

Answer 1

since velocity is 'change of distance with time' it is difficult to conceive 'instantaneous velocity' since if there is no time period there can be no displacement.

If you consider the velocity to be ds/dt then you can calculate a velocity for a given value of t

(I am not sure of the mathematics behind this - but the whole of calculus is based on infinitesimal changes, and has done us proud in solving physical problem so far... :-)