Someone unknown in the past found out that if the shadow was cast by a sloping object pointing to the celestial pole, it would cast a consistent shadow which would be in the same place at the same time every day. Practically everybody knows what a sundial is. Most people have a residual idea that, if they had to, they could make one. But most people's practical knowledge of sundials is confined to having seen some standard brass horizontal dials on plinths in gardens. They may even have one in their own garden, or perhaps in their garden shed because they don't know how to set it up.
Sundials measure time as it is.
Mechanical watches obviously cannot be made to run in this way. So Mean Time was invented, an artificial construct in which all days are assumed to be exactly 24 hours long. The sundials seen above church doors are a reminder of a time when sundials were the standard. Until about 200 years ago, public clocks could not be made sufficiently accurate to run for more than a few days without being reset, and the only way of resetting them was from a sundial. Until the railways came, there was no particular reason why people in, say, Bristol should keep the same time as people in London.