  The tight upper bound $\operatorname{pt}_+(G) \leq \left\lceil
\frac{\left\vert \operatorname{V}(G) \right\vert - \operatorname{Z}_+(G)}{2}
\right\rceil$ is established for the positive semidefinite propagation time of
a graph in terms of its positive semidefinite zero forcing number. To prove
this bound, two methods of transforming one positive semidefinite zero forcing
set into another and algorithms implementing these methods are presented.
Consequences of the bound, including a tight Nordhaus-Gaddum sum upper bound on
positive semidefinite propagation time, are established.

Combinatorics