The Employment Rights (Northern Ireland) Order 1996 has a set way of calculating continuity which overrides any agreement between the employer and employee. It depends on when the continuous period starts; when the continuous period ends; whether anything happens to break continuity between the start and the end; and whether any period of employment with another employer counts. The period of continuous employment begins on the date that the contract stipulates as opposed to the first day that the employee actually attends work. Likewise, the end date is the 'relevant date’ for the purposes of determining the employee’s length of employment rather than the last day actually worked.

There is a presumption that a person’s employment during any period is continuous. This presumption can only be rebutted upon the production of satisfactory evidence that there was a week of employment which did not count for the purposes of continuous employment, and continuity is not preserved by any provision in the Employment Rights (Northern Ireland) Order 1996. Therefore, any week or part of a week will count in computing the employee's continuity of employment if the employee’s relations with their employer are governed by a contract of employment during that time, irrespective of the hours worked, if any are worked at all, during the week.