This excerpt taken from the BP 20-F filed Jun 30, 2005.
Secondee shall take his vacation entitlement at times agreed with the Company.
entitlement to vacation remaining at the end of any calendar year may be carried forward to the next calendar year but no further. The entitlement to vacation (and on termination
of employment to accrued but unused vacation) accrues pro rata throughout each calendar year (disregarding fractions of days).
Bet you've never seen portfolio analytics like these.