Working with dates can get complicated when you need a time to change depending on the time of year for GMT timezones. Thankfully DateTime makes it easier.
First create a new instance of DateTimeZone and pass in the timezone.
Next create a new instance of DateTime and pass in tthe date and timezone, then all that is left is to format the date.
//set a date and time $sendDate = '2019-05-16 17:30:00'; //set the timezone $timezone = new \DateTimeZone('GMT'); //create a new instance of DateTime with the provided date and timezone $dateTime = new \DateTime($sendDate, $timezone); //format the date, in this case 2019-05-16T17:30:00 $sendDate = $dateTime->format('Y-m-d\TH:i:s');