<?php
add_action('after_setup_theme', 'mytheme_init');
function mytheme_init() {
  // Check if already saved the activation date & time
  // to prevent over-writing if user deactive & active theme
  // multiple time
  if(!get_option('mytheme_activation_time', false)){
    // Generate Current Date & Time in MySQL Date Time Formate
    $activation_datetime = current_time( 'mysql' );
    // Save it in `wp_options` table
    add_option('mytheme_activation_time', $activation_datetime);
  }
}