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titleFormat

Determines the text that will be displayed in the header’s title.

Date Formatter, default:

{ year: 'numeric', month: 'long' }                  // like 'September 2009', for month view
{ year: 'numeric', month: 'short', day: 'numeric' } // like 'Sep 13 2009', for week views
{ year: 'numeric', month: 'long', day: 'numeric' }  // like 'September 8 2009', for day views

As noted above, each view has a specific default. Get fine-tuned control with View-Specific Options. A single string alone will set the value for all views.

For views that display date-range text, to control the text between the two dates, customize titleRangeSeparator.