Return the locations (indices) of labels in the index.
As in the asof function, if the label (a particular entry in
where) is not in the index, the latest index label up to the
passed label is chosen and its index returned.
If all of the labels in the index are later than a label in where,
-1 is returned.
mask is used to ignore NA values in the index during calculation.
An Index consisting of an array of timestamps.
Array of booleans denoting where values in the original
data are not NA.
An array of locations (indices) of the labels from the Index
which correspond to the return values of the asof function
for every element in where.