Adding a time field for opportunity end date


I’m not sure if I’m on the right place…
I am now working on the customization of the opportunity form.
There is a mandatory field that is “end date”.
But I also need to get the end time.
As I can understand, I can only add a new field of type “Date time”, and not “just time”.
SO I added a new field called “end opportunity” which is of type “date and time”.
And I was wondering if it was possible to automatically get the “date” of this field and add it into the mandatory end date that can’t be removed ?
Do you see what I mean ?
Basically, the user will just fill the date once (in my custom field for example) and this value has to be filled in the other mandatory field.
Is it possible to achieve ? Or is it something better to do ?
Could you please help me ?

Thanks !

Hello, I as wondering if someone could help me.
I’m sure it’s something easy to solve, but I’m still a newbie and stuck with this thing…