Clicking the Add icon (the plus sign-shaped icon at the top right of a date) opens the screen for adding a record.
Date and time values will be entered in the field you specified for the "Date" in the Calendar view settings, regardless of the default settings (see below).

• If a "Date" field is specified for "Date Field":
  The date value of the calendar cell where you clicked the Add icon (the plus sign-shaped icon) is entered.
• If a "Date and Time" field is specified for "Date Field":
  The date value of the calendar cell where you clicked the Add icon (the plus sign-shaped icon) and the time that you clicked it are entered.
• If a "Created datetime" field or "Updated datetime" field is specified for "Date Field":
  Created datetime or updated datetime is automatically entered.

If you click the Add Record icon (the plus sign-shaped icon at the upper right of the screen), the default value of the field you specified for the "Date" field is entered.