On 10 April ekathimerini reported that it would be operational the following week for 2017 submissions so I assume it has been open since at least 20 April, possibly earlier. Cut off was reported then as the end of June but sometimes there are extensions.
An article in ekathimerini reports that the deadline for submitting income tax returns has been extended to 26 July but the deadline for payment, either in full or for the first instalment, remains as 31 July.
I wouldn't delay too long. My gut feeling is that the system does not have the capacity to process the number of submissions it needs to process by 26 July and that it will get increasingly hard to make a submission as the deadline approaches. "Plan ahead" is not a strong trait in Greece. Given an extension, Greeks will endeavour to make full use of it.