The defaultDate/default_date prop has a null or empty string value by default. You can pass an ISO date string (preferred rails method) or date object (preferred JS method) if you want a default value on page load. Use Ruby UTC DateTime objects and convert them to ISO date strings with DateTime.now.utc.iso8601.

If you use a Date object without UTC time standardization the Date Picker kit may misinterpret that date as yesterdays date (consequence of timezone differentials and the Javascript Date Object constructor). See this GitHub issue for more information and the anti-pattern examples below.