How to insert current date time in vscode?

As of Jan 2018 (release 1.20) you can use these new snippet environment variables.

Your example above would look like this:

"File Header": {
    "prefix": "header",
    "description": "Output a file header with the file name and date",
    "body": [
        "title: $TM_FILENAME",
        "date: $CURRENT_YEAR-$CURRENT_MONTH-$CURRENT_DATE $CURRENT_HOUR:$CURRENT_MINUTE:$CURRENT_SECOND",
    ]
}

Type head, press ctrl+space and it should show snippet menu.