// do shits here...
There is no method to get items sorted in descending order in firebase database. So you need to sort them by ascending and reverse the array in your programming language.
If you want to sort them by time you should be using timestamp instead of datetime as string. In this case you will get wrong ordering because they are actually compared as strings. For example: Imagine this two dates
2016-10-2T02:30:59.653Z. As dates first one is greater but as a strings (lexicographically) second one is greater.