A timer device adapted to monitor time periods associated with foods and other substances which may have a finite time during which they are suitable e for use, comprising a timer unit (8) capable of monitoring a plurality of time periods, each time period being associated with a tag (1) identifiable by a unique code such as an alphanumeric code. The tags (1) are attachable to a product such as a lid (2) of a food jar, and its identifiable code may be entered on a key pad (12) of the unit (8) such that the period during which the tag has been attached to the jar is monitored on the timer unit (8) and may be read at any time during the preselected period during which the product in the jar is usable. The device assists in ensuring safe use of products and reduces product wastage.