Calculation of start date calculation for ILR

Hello Admins,

I have a question on how the start date is calculated.

I changed from Tier 4 student to Tier 2 General visa in 2015 March through a premium appointment which was same day decision at that time.

Tier 2 Visa Issue date is 16th march.
Start date of Employment is 29th march.

I am now applying for ILR and my question is which date is used for 5 year continuous employment calculation ? Considering I can apply 28 days before 5 year completion , will I be eligible for applying based on 28 days before visa issue date or employment start date. ?

Thanks in advance

Tier 2 visa star date!!

That was quick answer CR001.

Thank you very much