If you have not filed income tax return till 31st August, and if no additional tax is payable, you can file the same till 31st March 2016 without an penalty and between 1st April 2016 till 31st March 2017with penalty. If some additional tax is payable then you might have to pay additional interest on such tax payable. This interest depends on the delay made by you in months and not on daily basis. e.g. paying additional taxes and filing return any time between 1st September and 30th September does not make any difference so far as your liability for interest is concerned.
Filing up of Income Tax Form may be simple. However, there are many provisions of income tax which you may not be aware of and may result in to tax liability. It is not like filling up any other application form where you are asked your personal details which you can easily fill on your own. There are some additional disclosures such as exempt incomes, names, PAN and share of co-owner of the property owned by you etc.
Some of the factors on which complications in filing Income Tax Return would arise depend on
1. Whether you are only salaried employee or also doing business.
2. Whether you have sold any assets such as house, shares, land during the year.
3. Whether you want to claim more benefits than what your employer has shown in salary certificate.
4. Whether you have any additional income which has not been shown in your form 16 given by employer.
5. Whether your 26AS shows any additional TDS, whether you own more than one properties, etc.,
6. Whether you have invested any amount in the name of minor from your taxable income.
7. Whether you have transferred any amount in the name of your spouse and he/she gets some income out of it.
8. Whether you want to claim refund as a result of excess tax deducted.
You are, therefore, requested to contact a Chartered Accountant in practice, who deals with filing of income tax returns and get your returns filed, if you want to be absolutely sure about the correctness of the details of income provided by you in the return.