This extension is known as a Patent term adjustment (PTA). Its intention is to accommodate for delays caused by the US patent office during the Prosecution of a US patent application. The total PTA is an addition to the 20-year lifespan of a US patent. The delays are broadly classified into 4 types:

• Type A - this delay is caused when USPTO fails to reply within the time period provided. As per US Patent law USPTO has to issue the First Office action within 14 months of filing of the application.All other office actions has to be issued within 4 months of receipt of an applicant response.Failing to this condition, type A delay will accounted.
• Type B - USPTO estimates the normal prosecution period of an application to be 3 years, i.e. between the date of filing of the application and the issue of the patent .If the prosecution exceeds this time, then Type B delay come into place.Type B delay will calculated if no RCE (Request For Continued Examination) is filed Prior to completion of 3-year period.If the first RCE is filed after 3-year period then type B delay will be calculated up to the date of filing of RCE.
• Type C - This type of delay is calculated in the events of secrecy orders or interferences.
• Applicant Delay - Applicant delay occurs when the applicant fails to respond to the office action within 3 months of mailing of an Office action(OA).

Calculation:

• Type A delay= Date of issuance of office Action -(Date of receipt of applicant response + 4 Months/14 Months)
• Type B delay=Date of issue of Patent/Date of filing of first RCE - (Date of filing of application + 3 years)
• Total PTA[12] =Type A+Type B + Type C - Applicant Delay -overlapping delays