Currently the MQTT Integration supports various component types (number, sensor, binary_sensor, text, …) for discovery.
Unfortunately “time” or “datetime” are not among the available.
I would like to request a mqtt.time / mqtt.datetime component, so I can control my heating vacation mode via mqtt using a datepicker.
Related: MQTT Discovery - input_datetime
Edit: also related: MQTT Homeassistant Support for "datetime" and "date" Fields · Issue #1120 · john30/ebusd · GitHub