How to preserve selected time when date only is changed?

[NaomiN]

I posted this question on stackoverflow https://stackoverflow.com/questions/48650305/using-bootstrap-datetimepicker-in-angularjs-directive

It seems when I select the date from the picker, the time portion adjusts to the current time. I don't want this behavior, I want the time to be unchanged in this case (if already set). However, I also want to be able to manually change time (in input and in the picker). Is there a way to achieve this behavior of not changing the time when date is changing?