It shouldn't vary between systems. Per the documentation of localtime, "$wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday." Sunday should universally be 0 and Saturday should be 6.

In the test, I set the list of weekdays to not be the default M-F to verify that nonstandard ranges are supported, but didn't think to mention that in the initial post. Sorry about the confusion there.