"Calendarius" and "calendarium" both have Latin roots and are related to calendars, but they have slightly different meanings.

Calendarius: This term typically refers to something related to the calendar, often in an adjectival sense. For example, "calendarius" might describe something that is of or pertaining to the calendar. It could also be used as a noun to refer to a person who deals with calendars or calendar-related matters. However, "calendarius" is not as commonly used or recognized as "calendarium."

Calendarium: This term usually refers to a calendar or a schedule, especially in a more formal or official sense. It can also refer to a treasury or account book, depending on the context. In modern usage, "calendarium" might be understood as a calendar, diary, or planner.

In summary, while both terms relate to calendars, "calendarius" is less common and often used in a more abstract or adjectival sense, whereas "calendarium" is more commonly used to refer to an actual calendar or schedule.