When determining how an aspect of your job measures against a given standard, your employer may use a rolling reference period. This means keeping a continuous current count, so you're always using the most up-to-date data. For example, if the employer is checking the hours you work against the Working Time Directive limits, it works out your average hours over the previous 17 weeks. So, this average is first calculated after your first 17 weeks of work. If it's a rolling reference period, when you complete week 18, your employer will take into account your average hours in weeks 2 to 18, and in week 19 the average of weeks 3 to 19, and so on.
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