Dear Jaideep,
Everyone who goes through the Payment of Gratuity Act,1972 for the first time, particularly the method of calculation described u/s 4(2) of the Act, would invariably encounter the doubt you have expressed. But, this is a question of law already well settled by the Supreme Court of India and several High Courts.
Even though the term 'gratuity' conspicuously escapes a definition under the Act, by the very scheme of the Act, it is very well recognized as a statutory terminal benefit to be conferred on the exiting employee by the employer based on certain minimum length of blemishless service under the same employer and his last drawn wages.
The exiting employee's last drawn wages may be calculated and paid by the employer in terms of time period spent by the employee on the job like daily, weekly, fortnightly and so on subject to a maximum of one month which is called as "the wage period" indicating the periodicity in which wages are paid as per the preexisting laws relating to payment of wages and fixation of minimum wages or the average earnings during the last three months under the piece rate scheme of payment of wages. Thus, the most convenient unit of calculation of time spent on work at the maximum would be a month which is universally considered to comprise of 30 days not withstanding the difference in the number of different Calendars either based on the rotation of the Moon or the Sun. But, no one can dispute the fact that every employee gets a minimum of 4 weekly holidays in a month. The wages for such weekly holidays is included in the remaining working days which is considered notionally as 26 days in a month. Therefore, 15 days wages for every completed year of continuous service set out for calculation of gratuity u/s 4 (2) is reasonable as per the explanatory clause to that sub-section.
That's why the Supreme Court categorically held in Shri Digvijay Wollen Mills v. Mahendraprataprai Buch [ (1980) 4 SCC 106 ] that 15 days' wages should be calculated by treating the monthly wages as wages for 26 days and not as wages for 30 days. Affirming the above decision, the Supreme Court in its judgment in Jeevanlal v. Appellate Authority [ (1984) 4 SCC 356 ] held further that the ascertainment of daily wages from monthly should be by dividing the monthly wages by 26 and gratuity should be calculated by multiplying the quotient by 15 and not by 13.
Hope it is clear to you now.