What is the name of the fictional calendar used in the Shire in J.R.R. Tolkien’s writings?

The fictional calendar used in the Shire in J.R.R. Tolkien’s writings is called Shire Reckoning.

Tolkien describes the Shire calendar in the appendices to The Lord of the Rings. Shire Reckoning divides the year into twelve months of thirty days each, with additional days arranged outside the ordinary months. The calendar’s structure reflects the Hobbits’ local customs and their adaptation of the older calendar of the Dúnedain.

The familiar dates 22 September and 25 March are especially important in the story. Bilbo and Frodo share 22 September as a birthday, while 25 March marks the destruction of the One Ring. Readers sometimes confuse Shire Reckoning with the calendars used by Gondor or with Tolkien’s modern dating conventions, but the Shire has its own named system.