3 added 114 characters in body
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Try This :-

$start = date("Y-m-d h:m:s", strtotime(2019-3-1)); //Pass 1st month Starting Date
$end   = date("Y-m-d h:m:s", strtotime(2019-3-29)); //Pass 1st month Ending Date

$collection=$collection->addFieldToFilter('created_at', ['gteq' => $start])
                       ->addFieldToFilter('created_at', ['lteq' => $end]);

Try This :-

$collection=$collection->addFieldToFilter('created_at', ['gteq' => $start])
                       ->addFieldToFilter('created_at', ['lteq' => $end]);

Try This :-

$start = date("Y-m-d h:m:s", strtotime(2019-3-1)); //Pass 1st month Starting Date
$end   = date("Y-m-d h:m:s", strtotime(2019-3-29)); //Pass 1st month Ending Date

$collection=$collection->addFieldToFilter('created_at', ['gteq' => $start])
                       ->addFieldToFilter('created_at', ['lteq' => $end]);
2 added 38 characters in body
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Try This :-

$collection=$collection->addFieldToFilter('created_at',array('neq'=>array('from' ['gteq' => $start$start])
                       ->addFieldToFilter('created_at', 'to'['lteq' => $end)$end]);

Try This :-

$collection=$collection->addFieldToFilter('created_at',array('neq'=>array('from' => $start, 'to' => $end));

Try This :-

$collection=$collection->addFieldToFilter('created_at', ['gteq' => $start])
                       ->addFieldToFilter('created_at', ['lteq' => $end]);
1
source | link

Try This :-

$collection=$collection->addFieldToFilter('created_at',array('neq'=>array('from' => $start, 'to' => $end));