When two calendars are linked, the 'Parent Calendar' is controlling the current date and time of the 'Child Calendar'. When the Parent Calendar changes date, that alters the date and time of the Child Calendars connected to it. If you have two different societies that keep track of time in different calendars, you can easily get the same relative date.
This also takes day and hour lengths into account, so if the two calendars are both on the same planet, we recommend keeping the hours per day and minutes per hour the same between them.
Parent and Child Calendars
One Parent Calendar can have any number of Child Calendars, but Child Calendars cannot have any Child Calendars under them.
Child Calendars cannot change their own date. Their dates only change when the Parent Calendar updates its date.
Once you have initiated a link between two calendars, some settings on both calendars will be disabled from being edited.
Things like month lengths, weekdays, leap days, hours, minutes, and eras are structural to a calendar, and changing them while two calendars are linked would be like changing the wheels on a moving car.
To change a parent or Child Calendar's structure, simply unlink it from any other calendar(s).
Relative Start Date
When you are creating a link for the first time, you will have the option to set the relative start date for the Child Calendar.
For example, if you have two gregorian calendars, and the Parent Calendar's date is the 1st of January, 2020. If you set the relative start date to be 1st of January, year 2020, the Child Calendar's date will now be 1st of January, year 1. This means that the Child Calendar "started" on the 1st of January, year 2020 relative to the Parent Calendar.