What is Todo.Txt and how to use it?

Todo. Txt uses the x for the same purpose. The date next to the x is the completion date, i.e., when you completed the task; due dates can be added to the end of a task in the due:YYYY-MM-DD format.
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Txt apps allow you to sort the list in different ways,

1. By priority.
2. By project.
3. By context.
4. By completion date.