For most people, you simply add up your working time over the last 17 weeks and then divide by 17. In the jargon of the Working Time Regulations 1998 (WTR), this is a '17week rolling reference period If you were sick or on leave during the last 17 weeks, you start the reference period a week earlier, and make up the missing days from your last week (i. e. the 18th week from