To me (not an expert), the issue appears to be in gad-data-model.php, line 158:
$current_date=$start_date_ts + (60*60*24*$count);
I believe $start_date_ts is not the correct value, might be a scope issue.
I replaced this code with
$current_date= time() + (60*60*24*($count-30));
this basically reacalculates $start_date_ts as a part of the $current_date calculation