How to format Vuetify data table date column?

You should use a custom row cell :

<v-data-table :headers="headers" :items="logs">
  <template v-slot:item.createdOn="{ item }">
    <span>{{ new Date(item.createdOn).toLocaleString() }}</span>
  </template>
</v-data-table>

I found out a way to format cell values using dynamic slot names and a function in the header object:

In the <v-data-table> I did:

<template v-for="header in headers.filter((header) => header.hasOwnProperty('formatter'))" v-slot:[`item.${header.value}`]="{ header, value }">
    {{ header.formatter(value) }}
</template>

and in the vue data property I did:

headers: [
    ...
    { text: 'Value for example', value: '10000', formatter: formatCurrency },
    ...
]

And finally in the methods prop I did:

formatCurrency (value) {
    return '$' + value / 100
},

Here's a sandbox to see it in action: https://codesandbox.io/s/vuetify-datatable-value-formatter-jdtxj

EDIT: In this specific case you could use momentjs or javascript's Date()