In Hartley v King Edward VI College, the Supreme Court held that if teachers lawfully strike for one day, the employer can make a deduction of 1/365 of annual pay, unless the contract of employment provides otherwise. The decision of the Supreme Court was unanimous.
The College made a deduction of 1/260 on the basis that there were 260 working days in a year. The teachers argued that their working hours were not limited to teaching hours as they regularly carried out their duties in the evenings and at the weekends.
This decision is specific to teachers though the principle may be applied in other industries where working days are not ascertainable.