I have 3 years 9 months of experience but since notice period for my organization is 3 months so I calculated that I will be having 4 years of experience on my last working day. I have mentioned in my resume that I have 4 years of experience . I got Job Offer from different organization which expected me to join in one month. So I negotiated with my higher officials for setting up my last working day within 1 month. This makes my experience as 3 years and 10 months.How should I explain this to new employer?
It's very easy to explain indeed: it's just a rounded up number.
Nobody's asking you how many days, hours, and minutes you worked.
For all intents and purposes 3 years 10 months is the same as 4 years.
Edit: Of course, as per one of the comments below, if you're prompted for a month count, then you have to be rigorous of your current count of months worked. If you're only asked for a year count, then it's fine.
First of all, never count future dates as part of your experience, that's a lie. You never gained the experiences from the future dates, on the date you are claiming to have the experience.
You are supposed to present the existing experience, not the probable future one based on some random assumption.
That said, in the current scenario, a 2-month gap would not be much of a problem, given you satisfied other criterion in the job description and cleared the interview. It can be seen as rounded-off or approximated.
However, next time onward, count only the experience you have. Will save you some troubled thoughts.
Why calculate your experience for the reader?
Quite frankly, the months don't matter. You are either a person with "almost 4 years of experience" or "more than 4 years of experience". 4 years precisely is only relevant on one single day.
Is calculating your experience a desirable piece of data? It seems very obtuse when you should have simply written:
Company XYZ: June 24, 2015 - Present
This would solve all ambiguity and avoid future issues.
Another reason not to perform the calculation ahead for the reader is because what if your resume is read a month after you send it so now you have 3 years and 11 months so your resume is underselling you? If you strongly desire to calculate your experience for the reader then you will have to add a date stamp to your resume so that they can perform further calculations.
At least for western companies, a few months of difference won't matter much. What counts the most is your capacity of delivery. Companies are more interested on your potential and energy you're bringing in rather than on how many months or years you stood 9 to 5 doing the same work over and over.
You should, however, make it clear this information difference as soon as possible. This way, you'll be able to assess whether the few months would cause any problems.