Ok,

It appears that pressing time and enter gets into some sort of program mode. The display shows 9900, then pressing any button underneath any of the numbers cycles through numbers 0-9 and letters like a u, and an upside down u and a c and a reverse c and the lower case t, and also a black display. I don't know what any of those mean or how I can get back out of this mode. I tried pressing time and enter again, but the enter button only turns on or off the PM indicator on the display.

Steve.