The date of application for a Tier 1 General visa, is either the date of submission of the application (for applications submitted in the UK by post) or the date that the associated application fee was paid (for applications submitted outside the UK or through the Home Office’s fast-track service).
The date of initial application dictates the criteria under which you will continue to extend the visa. It also dictates the criteria for which you will need to score points for an indefinite leave to remain application.
The date of your initial application may not be the same date that your visa permission was granted. For example, if you submitted your initial application by post on 15 March 2010 and your visa was granted on 20 May 2010. This would mean that you would continue to extend your visa permission under the Rules that were in place before 5 April 2010.