Django: Combine a date and time field and filter

I have a django model that has a date field and a separate time field. I am trying to use a filter to find a value on the latest record by date/time that is less than the current record's date time.

How do I use annotate/aggregate to combine the date and time fields into one and then do a filter on it?

models.py
class Note(models.model):
    note_date = models.DateField(null=True)
    note_time = models.TimeField(null=True)
    note_value = models.PositiveIntegerField(null=True)

def get_last(n):
    """
        n: Note
        return: Return the note_value of the most recent Note prior to given Note.
    """
    latest = Note.objects.filter(
        note_date__lte=n.note_date
    ).order_by(
        '-note_date', '-note_time'
    ).first()

    return latest.note_value if latest else return 0

This will return any notes from a previous date, but if I have a two notes on the same date, one at 3pm and one at 1pm, and I send the 3pm note to the function, I want to get the value of the 1pm note. Is there a way to annotate the two fields into one for comparison, or do I have to perform a raw SQL query? Is there a way to convert the date and time component into one, similar to how you could use Concat for strings?

Note.objects.annotate(
    my_dt=Concat('note_date', 'note_time')
).filter(
    my_dt__lt=Concat(models.F('note_date'), models.F('note_time')
).first()