Regional date formats vary throughout the world and it's often difficult to find a human-friendly date format that feels intuitive to everyone. The advantage of dates formatted like 2017-07-17 is that they follow the order of largest to smallest units: year, month, and day. This format also doesn't overlap in ambiguous ways with other date formats, unlike some regional formats that switch the position of month and day numbers. These reasons, and the fact this date format is an ISO standard, are why it is the recommended date format for changelog entries.

ISO 8601 specifies the use of uppercase letter T to separate the date and time. PostgreSQL accepts that format on input, but on output it uses a space rather than T, as shown above. This is for readability and for consistency with RFC 3339 as well as some other database systems.

This document includes an Internet profile of the ISO 8601 [ISO8601] standard for representation of dates and times using the Gregorian calendar.

Do I have to use the yyyy-mm-dd date format? Yes. That date format is defined by ISO 8601. It is an internationally accepted way to represent dates. If you don't like typing the date that way, use the due date and threshold date pickers in the task text editor.