Laravel Carbon date diffInDays() on string error

I need to find the difference between the two dates. Say i have 2017-02-01 - 2017-01-01. The number of days between the two days is the output

$formatted_dt1=Carbon::parse($a->date)->format('Y-m-d');
$formatted_dt2=Carbon::parse($c->dt)->format('Y-m-d');
$date_diff=$formatted_dt1->diffInDays($formatted_dt2);

If I give the above code I get the error as

FatalThrowableError in ReportsController.php line 67:
Call to a member function diffInDays() on string

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