Nixie tubes tend to develop a phenomenon called “cathode poisoning” on digits that are not in regular use. To make sure the digits in your clock stay in good condition, all of them need to be exercised regularly. This is done by running a “cathode poisoning prevention routine” regularly.
When writing your own code for a clock (or any other display with our R|Z568M nixie tubes), please use ratio 60s: 0.2s. For every 60 seconds when the tube is on (whatever digit), exercise every other digit for 0.2s. For instance, if you let your display run for 10 minutes (600 seconds) on digit “1” (this is the case of the leftmost tube in a typical clock), you need to run a routine that will let each other digit glow for 2 seconds (60*10: 0.2*10).