I was able to insert data to my sqlserver database using this:
date("Y-m-d H:i:s", strtotime($promostart));
So the value on my database is: 2021-08-10 23:59:00.000
I tried echoing the value from my database directly to my input field like this: <input type="datetime-local" id="dateStart" name="dateStart" value="<?php echo $row[DurationStart];">
But the input field does not show both time and date specified in the value. and shows up like this: 
What should i do to be able to output/display it to my datetime-local input field?
Any clue or guide is greatly appreciated. Thanks!