def filter_submissions(submissions, start_time, end_time = None, username = None):
    """Return all submissions created after the start_time.
        Optional: Also before end_time if given.
        Optional: Also by username if given."""
    filtered = []
    for s in submissions:
        if end_time and s.created_utc >= end_time:
            continue
        elif username and username != s.author.name:
            continue
        elif s.created_utc > start_time:
            filtered.append(s)
    return filtered