from datetime import datetime


def get_duration_between_datetime_strings(start_time: str, end_time: str) -> str:
    """Given two datetime strings, compute the duration between them

    Args:
        start_time (str): ISO format datetime string
        end_time (str): ISO format datetime string
    Returns:
        The duration between the time in string format
    """
    start = datetime.fromisoformat(start_time)
    end = datetime.fromisoformat(end_time)

    return str(end - start)