Extract the timestamp from a Nonce. To be sure the timestamp was generated by this site,
    make sure you validate the nonce using validate_nonce().

def get_nonce_timestamp(nonce):
    '''
    Extract the timestamp from a Nonce. To be sure the timestamp was generated by this site,
    make sure you validate the nonce using validate_nonce().
    '''
    components = nonce.split(':',2)
    if not len(components) == 3:
        return None

    try:
        return float(components[0])
    except ValueError:
        return None