Dating scan a sham. 😒

Jasmin
So, had my dating scan at what I thought was 12 weeks (due Christmas <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a>). The technician said I was 11 weeks and 6 days, due Christmas Day.
Based on the dating scan information, I would have been 13 weeks and 3 days yesterday when I called the same radiology clinic to organise the NT scan.
According to the clinic, however, their "worksheet" said I was 14 weeks yesterday (now due 22 December) and therefore no longer eligible for an NT scan.
Who am I meant to believe? What is the point of a dating scan if they're going to give me an inaccurate date anyway? 😒