Injecting external dependencies (requests, Now, etc)

In the PyCon talk, Tom mentions explicitly injecting external dependencies such as Requests or Now or RandInt.

In the example, he mentions defining Now as:
Now = typing.Callable[[], datetime.datetime]

and using it:

def add_billing_record(record: BillingRecord, now: Now) -> BillingRecord:
   """Below is just an example"""
    record.timestamp = now()

I have a custom method that I use to generate timezone aware datetimes. In the slide, he defines the type as a callable but it is not linked to any actual definition. Should I create a component with a __call__ method? Or is there another way to have the dependency injector call the correct code.