I have data like this (format yyyy-mm-dd):

start_date                end_date
2012-06-01                2012-12-31
2013-01-01                2013-12-31
2014-01-01                2014-12-31

I expected sum of each period between start and end date: 2 years 6 month
but i got : 0 years 30 month

this is my code

foreach ($data->historical_positions->where('is_valid',true) as $key => $value) {
                    $periods = CarbonCarbon::parse($value->start_date)->diff($value->end_date);
                    $periods_yr = $periods_yr + $periods->y;
                    $periods_mt = $periods_mt + $periods->m;
                }