CoS Start Date

I was granted Tier 2 visa for 3 years and one month against CoS issued for a period of 3 years. This Tier 2 visa is expiring on 15 th May 2015. My employer has issued CoS for extension for another period of 3 years, but the start date for this CoS is mentioned as 16 th may 2015 i.e . the next day of expiry of my current visa, instead of the end date of previous CoS. I am not sure whether the start date for new CoS should be the end date of the previous CoS or next day of expiry of the current visa. Whether this start date date is valid or it will affect my application and can the start date on the CoS be amended.

It is right.
cos is a document which gives ur period of stay and cos is valid from 3 months from the date of issue via sms
the period of stay always precedes ur last expiry date .

Thank you for the reply. Is it valid if CoS start date is mentioned as the end date of expiry of this visa i.e. 15 may 2015 in this case. Can I apply in person now at premium service centre or I can apply in person only 28 days prior to the end date of my current visa.

the start date on the new cos should be next date of your expiry cos date of previous cos and NOT the next date of visa expiry.
i think your employer should add a note to new COS by logging into SMS.