of a single unit.
(Intervals like 5 minutes, 6 hours, 10 days are possible). <code>pad</code> will
figure out the interval of the input variable and the step size, and will
fill the gaps for the instances that would be expected from the interval and
step size, but are missing in the input data.
Note that when <code>start_val</code> and/or <code>end_val</code> are specified, they are
concatenated with the datetime variable before the interval is determined.</p>
<p>Rows with missing values in the datetime var