A replaceable cap (10) for a transdermal liquid dosing device (12) such as an insulin pen is provided. The cap includes an elongate hollow body (14) with a first open end (16) which can be placed over a front part (26) of the dosing device and a second closed end (18) opposite the first end. The cap body also includes a cavity which opens into the interior (50) of the cap body and which houses a control unit (52) which includes a timer unit, a switch mechanism (56) that stands at least partially proud of the cavity, so as to project into the interior of the body, and a timer display unit (58) which displays time counted by the timer unit on an outer surface of the body of the cap. The switch mechanism is engaged by abutment of a surface of the front part of the dosing device when the cap is placed on the dosing device, and released when the cap is removed from the dosing device, the engagement and/or releasing of the switch mechanism causing the timer unit to reset after the elapse of a predetermined period of time, the time since the timer unit was last reset thereby indicating the time that has elapsed since the dosing device was last used.