I want to find out when my Windows XP PC's HDD was last partitioned.
Is there a partition table creation date?
A master boot record contains boot code (never changes), the four partition entries, and a signature or two. No dates.
Since the MBR can be modified after booting off a CD or even with another computer, I don't see any guaranteed (or merely likely) way you can deduce the date either. Running a disk/partition editor does not mean the partition table was modified.