Calcite implements EXTRACT, FLOOR, CEIL, CAST, +, - on date/time values and much can be accomplished with these. But there are other useful functions in other databases.
For example MySQL has DayOfWeek. See https://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html. It is tricky to achieve the same in Calcite (you'd need to subtract the epoch and take the interval modulo 7).
We need to review the date/time functions in MySQL, Postgres and Oracle, and add functions to ensure that you can accomplish the same things in Calcite fairly easily.