Dates in Pivottables

I have a workbook which generates 20 or so pivottables each with a different
query in Access XP and each time the workbook is opened the pivottable are
rebuilt by VB procedures triggered by ThisWorkbook. I have a year start and
year end parameter coded as "Private Const YearStart as Date = #1/4/2006#"
and "Private Const YearEnd as Date = #3/31/2007#" and this arrangement work
fine....On my machine. But install this workbook on a coworkers laptop and
its appears to mix the data formats eg. start date is day/month/year and end
date keep changing to month/day/year format. Both machine have the same
regional settings for days.

Any pointers as to the cause??
TIA
Johnb