regression-1/date-calc

[OkayAlright]

To cover what the previous version of this tool could do, there needs to be some generic way to calculate the different in days between two dates given a CSV row.

It should allow for any date format (passing in a string of this format would do).
There also need to be a way to point it at any indexed column, not a fixed one (look at the filter indexing to see how to do this).