These docs are for an old release. Info on upgrading to v4
edit doc

slotLabelFormat 2.4.0

Determines the text that will be displayed within a time slot.

format string, default: 'h(:mm)a'

The default value depends on the current locale.

The default English value will produce times that look like “5pm” and “5:30pm”.

Timeline View

For Timeline view, one string can be given for creating a single header row, or an array of strings can be given to create multiple tiers of header rows.

slotLabelFormat: [
  'MMMM YYYY', // top level of text
  'ddd'        // lower level of text

Only available with the Scheduler plugin.

Prior to v2.4.0, this setting was named axisFormat.