# MATLAB: Changing time from decimal days to +- 0 from high water

logical?positive integer

I am attempting to plot pressure gradient over a tide cycle. I would like to change the time, currently in decimal days, into +- hours with high water as 0.
%find high water value pkIndex = max(pf)
%find time value at high tide timepkIndex = tf(pkIndex)
%I experience this error Attempted to access tf(1.8268); index must be a positive integer or logical.

[~, pkindex] = max(pf);