If you make your declaration online then you have a little longer, and if you are already registered in the system and employed, then it could be just a case of checking the figures.
Your first port of call with any questions should be your Trésor Public as they will be able to supply you with the latest advice, you can search for Trésor Public offices here.
But what do you do about income you have received from overseas and especially if you need to convert it to euros?
Using figures from the Banque de France is acceptable and they supply details of the average for the euro exchange rate from the past two years.
Other currencies are available on the Banque de France website in a .csv format.