ILR (SET O) Tier 2 Application Query

Dear all moderators and others, I need to help with my ILR (SET O) Application (5 years in Tier 2 ). There is box that asks the following questions-
When did you first enter the UK? (Required)
This is the date you entered the UK when the period of stay you are basing this application on started.

Now, what would be the correct answer based on the fact that I first entered to the UK in 2006 with student visa and switched (inside the UK) to Tier 2 Gen in 2016? Many thanks in advance.

It does not matter which date you put in. That question is NOT important