Roz, seniority rules are listed in your Regional Supplement (not in the Master portion of the Contract.) What Supplement do you fall under?
Be aware some areas may require more than 30 days to make seniority, or have periods in the Winter and Summer called "free periods" when seniority is not earned because the jobs are declared "temporary" during Peak, or "vacation cover" during the Summer.
UPS Master Agreement & Regional Supplements:
http://tdu.org/node/1617