Time is measured in [a]eons, millennia, centuries, decades, years, quarters, months, weeks, days, hours, minutes, seconds, and trices (or more recently milliseconds, microseconds, nanoseconds, ...)
For the complex relationship between dates and days and festivals such as Easter, have a look at The Perpetual Calendar. For the period of several hundred years when dates and even years were different in different countries all purporting to date from the birth of Christ, see Old Style and New Style dates.
Both postal history and family history can benefit from knowledge of what day of the week and month (and what year) a certain day was in a particular country.
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