Hours, minutes and seconds are in hexadecimal system. This is sexagesimal format according to Wikipedia. In Plasma4 the format was BCD (Binary-coded decimal) which means that each digit was encoded separately in decimal system.

My fix allows users to choose the format in config. By default the sexagesimal format is used since that was the format after porting the widget to Plasma5.

Here's a screenshot of 2 widgets. On the left there's Binary Clock widget after choosing BCD format in config and on the right there's an old widget displaying time in hexadecimal form. Screenshot was taken when my Digital Clock showed: 00:19.