"Total knee replacement cancelled"

I have waited 18 weeks for knee replacement which should have been done on the 8th Dec but it was cancelled on the 7th Dec .I have been in touch with the admissions booking team who cannot give me a further admission date they say it's most likely to be at the end of January 2016. I thought the NHS policy was to be treated within 18 weeks after seeing their consultant. Not knowing when you will get an admission date is really annoying .