Using mariomka/vue-datetime as component for Vue-Formly

app.js

import Vue from 'vue'
import App from './App.vue'

import VueFormly from 'vue-formly'

// Adding it to VueFormly
import datepicker from "./Datepicker.vue"
VueFormly.addType('datepicker', datepicker);

// Use VueFormly globally
Vue.use(VueFormly)

Component.vue

<template>
  <form v-on:submit="save()">
    <formly-form :form="form" :model="model" :fields="fields"></formly-form>
  </form>
</template>

<script>
import modal from 'vue-strap/src/Modal'

export default {
  name: 'calendar',
  data () {
    return {      
      form: {},
      model: {
       startDate: '',
      },
      fields: [
        {
          required: true,
          key: 'startDate',
          type: 'datepicker',
          templateOptions: {
            text: 'Start Date'
          }
        }
      ]
    }
  }
}
</script>

Datepicker.vue

<template>
   <div class="form-group formly-datepicker" :class="[ to.inputType, {'formly-has-value': model[field.key], 'formly-has-focus': form[field.key].$active, 'has-error': hasError}]">
      <label>{{field.templateOptions.text}}</label>
      <datetime v-model="model[field.key]"
        placeholder="Pick a date"
        type="datetime"
        input-format="DD-MM-YYYY h:mm:ss"
        monday-first
        auto-continue></datetime>
      
      <error-display :form="form" :field="field.key"></error-display>
   </div>
</template>

<script>
import Vue from 'vue'
import baseField from 'vue-formly-bootstrap/src/fields/baseField';
import Datetime from 'vue-datetime';
Vue.use(Datetime);

export default {
   mixins: [baseField]
}
</script>

Register the component locally feels better for this use case.

export default {
  mixins: [baseField],
  components: {
     datetime: Datetime
  }
}