On this topic, there are a hundred answers in the ethers, but unfortunately most of them are wrong.
This still applies this year, anyway. For instance, your 2017 tax return must be submitted by May 31, 2018. The due date is extended to July 31 of the following year starting for your 2018 tax return.
If your tax returns are submitted by a tax consultant such as our office, the deadline is further extended without additional conditions to February 21 of the second following year. For example, your 2018 tax return is due by February 21, 2020. For 2017 you still need to apply for a one-time extension to extend the due date to February 28 2019. If not, your 2017 tax return is due by December 31, 2018.
This due date should be taken seriously to avoid late surcharges and penalty payments!
By the way, contrary to popular custom, the extended due date allowed for tax consultants is intended to facilitate distribution of your business activities across the entire year. Many taxpayers misuse the due date to submit their documents just before the deadline which means tax consultants unfortunately come under time-limit pressure despite the extension.