50 There is nothing in s 51 of the PSMA or in the delegation of powers pursuant to s 51 made under s 15, which requires that the period for which the employee is to act be specified by reference to particular dates, rather than in some other way. Indeed, it will often be more convenient that it not be so specified. It appears to me that a direction to act "while the chief executive officer is on annual leave" or "from today until the chief executive officer is recovered from illness and returns to work" or "while the chief executive officer is attending a conference in Sydney" would all be directions which would specify an identifiable period. The period would be identified, not by reference to the calendar, but by reference to whether particular events had occurred. The convenience of such directions in terms of the efficiency of public administration is obvious. Holidays, illnesses, conferences, and no doubt other events, may be cut short or prolonged in an unforeseen manner, and a direction the duration of which is specified by reference to those events, avoids potential gaps in administration. It is true that there is a risk that such an event may, for unforeseen reasons, continue for longer than the period of three months specified in the delegation. Should that be so, the terms of the delegation pursuant to s 15 of PSMA would serve to limit the direction, so that it would cease to have effect upon the expiry of the three-month period.