I'm looking to update a large package (with subpackage files called in the same context) to be compatible with version 10. A lot of the functionality surrounds plotting, particularly some custom variants of `DateListPlot`. To get my desired `PlotRange`s, ticks and date label formats, the package does a lot of manipulation of dates, so it hits the change in the treatment of `DateDifference` [discussed here](http://mathematica.stackexchange.com/a/54573/8).

I wanted to just use the following in the preamble of the package, i.e. between the `BeginPackage` statement and <code>Begin["``Private`"]</code> (ignore that double backtick, you know what I mean).

    If[$VersionNumber >= 10,
    SetSystemOptions["DataOptions" -> 
     {"ReturnQuantities" -> False, "ReturnEntities" -> False}]
    ];

However this does not actually set the system options. Loading the package and then evaluating

    SystemOptions["DataOptions"]

Returns:

    {"DataOptions" -> {"ReturnEntities" -> True, 
      "ReturnQuantities" -> True, "UseDataWrappers" -> True}}

The alternative to this would be to add a lot of `If[$VersionNumber...]` hackery for multiple functions inside the package (and handle `DateObject` input for each of my custom plot functions, sigh).

So is there any way to set `SystemOptions` when a package is loaded?