Bclocks

In BCD clocks, the hour tens column usually uses 2 bits (values 1 and 2) because hours only go up to 2. So if the "2" is lit in hour tens, that means 2. Hour units: 5. So hour = 25? No — 25 is impossible. Aha — BCD for time is interpreted as two separate decimal digits: The tens digit (0-2) and units digit (0-9). So if hour tens = 2 and hour units = 5, that means 25 in decimal, which is a valid 24h time (25 = 1 AM next day). But most clocks cap at 23. Thus, for hours 00-23, the tens digit is 0,1,2. When tens=2, units can only be 0-3. So a proper BCD clock would never show hour units=5 with tens=2. So the example 25:26 is actually an invalid display — proving that the reader must understand the constraints.