Is it OK to send email to journal asking for extension for a revision? I am in the process of sending the response to reviews to a journal. It looks like it will take some days for the revision. 
In the reviews, the associate editor had commented the following 


  Finally, I would appreciate if you could submit your revised paper by Jan 12, 2018. If you cannot meet this deadline please contact us (xxx@elsevier.com) to request an extension, otherwise your paper may be withdrawn.


I was wondering if it was OK to write to them asking for an extension or should I somehow submit the revision by Jan 12?
Ask for an extension. This is quite common in case more extensive changes/analyses need to be done.

Submitting a subpar version would waste their and their reviewers’ time, and may even end in a desk rejection for you.

When asking for an extension, shortly explain why you need more time (ideally, related to the reviews you got – show them you are working on the paper), and make a good estimate on when you will be able to resubmit. You don’t want to ask for several extensions during the same round.