Magento 2 convert date to timestamp using best practice.

Magento 2 You can Converts input date into GMT timestamp using Magento\Framework\Stdlib\DateTime\DateTime Class.

You can set easily convert timestamp using core PHP by function strtotime().
like, strtotime(‘+1 years’);

Using Magento You can Converts input date into GMT timestamp by using Magento best practice way by injecting class dependency in the construct method.

<?php
/**
 * @var \Magento\Framework\Stdlib\DateTime\DateTime
 */
protected $dateTime;

public function __construct(
   Magento\Framework\Stdlib\DateTime\DateTime $dateTime,
) {
   $this->dateTime = $dateTime;
}

/* Convert date to timestamp usign Magento 2 Best way */
public function convertGmtTimeStamp()
{
    $dateToTimestamp = "2019-05-20";
    $timeStamp = $this->dateTime->gmtTimestamp($dateToTimestamp);
    return $timeStamp;
}

call from template or any other function,
echo $this->convertGmtTimeStamp();

Output in timestamp:
1558310400