Apart from the useful info already there in the above posts - I would also like to contribute a detailed treatment of
the sections in the Act from which we can infer that 240 days of service should be considered as one year of continuous service.
the definition of the term “continuous service” in Section: 2 Definitions is as below
Definitions.(b) “completed year of service” means continuous service for one year;
 [(c) "continuous service" means continuous service as defined in section 2A;]
Section 2A itself is defined as below
For the purposes of this Act, -
(1) an employee shall be said to be in continuous service for a period if he has, for that period, been in uninterrupted service, including service which may be interrupted on account of sickness,
accident, leave, absence from duty without leave….
(2) where an employee (not being an employee employed in a seasonal establishment) is not in continuous service within the meaning of clause (1), for any period of one year or six months, he shall be deemed to be in continuous service under the employer -
(a) for the said period of one year, if the employee during the period of twelve calendar months preceding the date with reference to which calculation is to be made, has actually worked under the employer for not less than -
(i) one hundred and ninety days, in the case of an employee employed below the ground in a mine or in an establishment which works for less than six days in a week; and
(ii) two hundred and forty days, in any other case;
As per this definition, 240 days of service should be considered as one year of continuous service
I have discussed this issue in detail at my blog
24th January 2011 From India , Bangalore