UK Visa Entry - Start Date

[jirstany]

Hi,

I will need to be applying a new UK Visa ( Tier 2 General Category) mostly for the employment secured with a company in the UK. I will be applying for a Visa from India.

Since there's the 30 day entry clearance visa Vignette stamped on the passport. Here's my question/query in order for me to be able to apply promptly to avoid delays.

How's the start date - derived? - ( PS - I know it's 2 weeks prior to COS Start Date)

Case:
Current Date - 13-Mar-2018
Visa Application Date - 20-Mar-2018
CoS Start Date - 30-Apr-2018.

If i Apply for the visa as on 20-Mar-2018, assuming if it at max takes 10 days in case of normal application or 5 days in case of Priority Application process, will the start date of the 30 days entry clearance still be 15th April ( going by cos Start date as 30-Apr-2018) & will be get back my documents & passport as of 30-Mar-2018 ( in case of normal process) or 26-Mar-2018 ( in case of Priority Application Process) with Visa Start date as 15th-April?

If not, can you confirm of how the Start Date is arrived?

[jirstany]

Any responses from the admin will be greatly appreciated.