UPS Xml Sunday only Bug

As weird as that sounds I found a bug in the UPS XML module that seems to only occur on Sundays? Looking at the code I do not see anything specific to a day of the week so I'm a little confussed on how to debug this or even recreated it.

What is happening is that on 2nd Day Air service 13 customers spanning a 4 month period were only charged $2.00 for shipping. Also two customers within the same time frame were only charged $3.00 for ground service.

So in total 13 orders and the only thing I can find that they have in common was all of them were placed on a Sunday.

Here are the dates:

2nd Day Air Service Orders:
Sunday 28 June, 2009 - 1 Order
Sunday 12 July, 2009 - 3 Orders
Sunday 19 July, 2009 - 1 Order
Sunday 30 August, 2009 - 4 Orders
Sunday 13 September, 2009 - 2 Orders

Ground Service Orders:
Sunday 12 July, 2009 - 1 Order
Sunday 13 September, 2009 - 1 Order