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.