A watch case back cover assembly includes a case back cover removably mounted to a case frame, an elastic member, and a cover plate. The case back cover includes a recess defining an outer side thereof, wherein the recess is defined by a bottom surface and an annular periphery, and wherein the annular periphery has an annular groove defined therein. The elastic member includes at least one elastic piece slidably received in the annular groove and at least one elastic extension bearing against the bottom surface. The cover plate it is secured to the elastic member to rotate therewith and has characters formed on an outer face thereof. The cover plate and the elastic member are rotatable relative to the case back cover when the cover plate is pressed.

A new and improved shock-resistant electronic watch module is provided, which comprises a case, a frame of electrically insulating material mounted in the case, a lens plate of an electrically insulating material being flexibly mounted on a first side of the frame, a substrate mounted to the lens plate, a frequency standard, a frequency divider, an electro-optical display means, and means for driving the display, all being mounted to the substrate, and, a battery removably mounted on a second side of the frame and being electrically coupled to the substrate.

A system for measuring the accuracy of an internal timing source of a device having a light emitting display which is energized at a rate derived from the timing cycle of that timing source includes an optical pickup disposed for receiving light emission from one element of the display and generating a signal having the same frequency as the frequency of such light emission. In one embodiment, the pulses of that signal which occur within a given time period are counted and the number of such counted pulses is supplied to a display for human recognition. In a second embodiment, the time elapsed during one or more complete light emitting cycles is measured and displayed for human recognition. The pulses of a timing signal having a period of one microsecond which occur during one or more complete light emitting cycles are counted and the resultant count is displayed. A method includes the steps of comparing a time reference signal with a signal corresponding to the light emitting cycle of the light emitting display, determining the ratio of one parameter of the time reference signal to one parameter of the light emitting cycle, and displaying the value of either the ratio, a multiple of that ratio, or a submultiple of that ratio.