Write the email then inject this JavaScript on the page to schedule an email submission.

mailTimer.js

// No Gmail, o ID do botao sempre muda quando a pagina é atualizada
// Mudar o id na linha abaixo
btnElementID = '';
btn = document.getElementById(btnElementID);
intervalID = window.setInterval( function checkTime() {
   // https://developer.mozilla.org/pt-BR/docs/Web/JavaScript/Reference/Global_Objects/Date
   var send_on = new Date( 2015,11,8,6,0 );
   var now = new Date();
   if ( now.getTime() > send_on.getTime() ) {
     console.info( 'Mandei!' ); // Sent
     btn.click();
     window.clearInterval( intervalID );
   } else {
     console.log( 'Ainda não ( '+ send_on +' ) > ( ' + now +' )'); // Not sent yet
   }
}, 5000 );