What git command could be used to check when a remote branch was created?
Note: this answer shows how to find branches that have been *updated* a long time ago, not branches that have been *created* (i.e., spliced off of some parent) a long time ago. I believe this is what the OP actually wanted (as opposed to the title he chose for the question), as this would lead to good candidates for deletion.

You have to recall that a branch in git is nothing "physical". It is, by definition, and literally, only a simple text file in .git/refs which only has its file name (which is the branch name) and the hash of the tip/head commit. It has no date information or anything else.

So git branch will not help you here. 

The object type that has dates associated with it are commits, in git. So you have to list commits. This is done with git log. 

So a working command would be:

git log --remotes --before 2018-01-01 --no-walk --decorate


You can read up the details on https://www.git-scm.com/docs/git-log , but in short, it means 


--remotes: start with any and all remote branches (substitute --branches for local branches)
--before ...: only list commits older than the date given
--no-walk: only list the very first commit for each branch, don't walk back the history
--decorate: display all tag and branch names associated, just in case


This will give you all branch heads that are older than your given date.