Hello @kartik,
The date_format validator takes only one date format as parameter. In order to be able to use multiple formats, you'll need to build a custom validation rule. Luckily, it's pretty simple.
You can define the multi-format date validation in your AppServiceProvider with the following code:
class AppServiceProvider extends ServiceProvider  
{
  public function boot()
  {
    Validator::extend('date_multi_format', function($attribute, $value, $formats) {
      // iterate through all formats
      foreach($formats as $format) {
        // parse date with current format
        $parsed = date_parse_from_format($format, $value);
        // if value matches given format return true=validation succeeded 
        if ($parsed['error_count'] === 0 && $parsed['warning_count'] === 0) {
          return true;
        }
      }
      // value did not match any of the provided formats, so return false=validation failed
      return false;
    });
  }
}
You can later use this new validation rule like that:
'trep_txn_date' => 'date_multi_format:"Y-m-d H:i:s.u","Y-m-d"' 
Hope it helps!!
Thank You!!