Q. Re- your answer on the permissibility of the home use of inaccurate scales and other measuring devises such as baby bottles and measuring tapes, due in part to the fact that they are not used for commercial purposes. How about a clock that is used to determine the amount of time a cleaning person or caregiver has spent working at home. Would you transgress if it is not accurate? How exact does it have to be not to violate any prohibition?
A. Tuvcha Yabiu, (Hilchos Sh'cheinim, 55) maintains that an ordinary clock or watch is primarily a device for telling time. Therefore it is not included in the prohibition of keeping an inaccurate measuring instrument in one's house. Even when used occasionally for commercial purposes such as to determine the amount of time a cleaning person or caregiver has spent working at home, it is only a secondary use.
Horav Shlomo Miller Shlit'a further asserts, that people don't usually care about short periods of work time, either added or subtracted and it is uncommon that a clock should be more inaccurate than that. He adds that today it is also common to own a watch or cell phone that can corroborate the time in the clock.
The Rov stresses that this does not apply to commercial punch clocks used in many factories, businesses and similar, where each employee has to enter his card, chip or biometric ID to mark the time when he arrives and when he departs. The time recorded on the card or memory, will serve as the basis for calculating his wages. If this dedicated clock is inaccurate, he may be paid too little or too much and it has to be kept accurate.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a