I would say time of day is not completely arbitrary, but it also does not require exact alignment with physical phenomena to have meaning. For example, if one

I would say time of day is not completely arbitrary, but it also does not require exact alignment with physical phenomena to have meaning. For example, if one were to say it is 12pm, people would understand roughly where we are in the diurnal cycle. People will understand it is not night time. People will know roughly where the sun is in the sky. Rough information is still information. Information doesn't need exact boundaries and discrete rules to carry meaning.