From PHP.NET documentation:
"The valid range of a timestamp is typically from Fri, 13 Dec 1901 20:45:54 GMT to Tue, 19 Jan 2038 03:14:07 GMT. (These are the dates that correspond to the minimum and maximum values for a 32-bit signed integer). However, before PHP 5.1.0 this range was limited from 01-01-1970 to 19-01-2038 on some systems (e.g. Windows)."
Use instead 'date("Y-m-d", strtotime($datevar));':
$dt = new DateTime($datevar);
$dt->format('Y-m-d');
OR
(new DateTime($datevar))->format('Y-m-d');