TypeError: formatDate is not a function appears in Vue.js

TypeError:formatDate is not a function
Vue warn—Error in v-on handler: "TypeError: formatDate is not a function"
found in
--- ><BButton>
<Create> at src/views/Create.vue
<App> at src/App.vue
<Root>

<section>
    <h2class="title is-3">title</h2>
         <b-input type="text" class="new_input" placeholder="Title" v-model="title">/b-input>

         <b-input type="text" class="tag" placeholder="tag" v-model="tag"@keypress.enter="addTag">/b-input>
        <div v-if="tags.length>0" class="tags">
          <lib-for="(tag,idx)intags":key="idx" class="content is-rounded">
           {{ tag}}
           <span class="cursor-pointer" @click="removeTag"></span>
          </li>
        </div>
        <textarea class="textarea" placeholder="Content" v-model="content">/textarea>
      <b-button type="is-primary"@click="publish" expanded>Post </b-button>
  </section>
    
    <script>
    import firebase from 'firebase';
    import {auth} from '@/main'
    import {db} from '@/main'
    export default {
      data(){
        US>return{
          title:',
          tags: [ ],
          content:',
          tag:',
          currentUser: {}
        }
      },
      created(){
        auth.onAuthStateChanged(user=>{ 
          This.currentUser=user
        })
      },
      methods: {
        addTag(){
          this.tags.push (this.tag),
          This.tag='"
        },
        removeTag(idx){
          This.tags.splice(idx,1)
        },
        publish(){
          const date = this.$date(new Date(), "ddMMMM,yyy")
          db.collection('posts').add({
            title —This.title,
            tags —this.tags,
            content —This.content,
            createdAt:date,
            uid —this.currentUser.uid
          })
          .then(post)=>
            this.$router.push('/post/'+this.post.uid+'/'+this.post.id),
            alert('The post got published!')
          )
        }
      }
    }
    </script>