Filter log groups by their creation date.

    Also sets group specific value for start to the minimum
    of creation date or start.

def filter_creation_date(groups, start, end):
    """Filter log groups by their creation date.

    Also sets group specific value for start to the minimum
    of creation date or start.
    """
    results = []
    for g in groups:
        created = datetime.fromtimestamp(g['creationTime'] / 1000.0)
        if created > end:
            continue
        if created > start:
            g['exportStart'] = created
        else:
            g['exportStart'] = start
        results.append(g)
    return results