That interpretation is not even close to what the government has said
"Johnson will underline that each stage is subject to four tests – the successful continued deployment of the vaccine programme, continued evidence that vaccines are reducing hospitalisations and deaths, low infection rates which do not risk a surge in hospitalisations, and that the programme is not threatened by a new Covid variant."
So why are the dates there then? A data-driven approach is When (Condition A is met) Then (Action B is taken). The conditions referred to are subjective assessments, not data points.
