Timewarrior stores intervals, most of which are closed, and only one which may be open. These intervals (inclusions) interact with exclusions (holidays etc) and the result is complex.

As an example, support that Saturday and Sunday are defined as exclusions, and an open interval is created tracking work on a Friday afternoon.

Until that interval is closed (`$ timew stop`

), it remains open. On the
following Monday there is still one open interval stored, but if a report is run,
that real interval is now represented by two separate intervals: Friday afternoon
and Monday morning. This is because of the weekend exclusions. These two intervals
are called synthetic intervals, and are related to the original real interval.
Synthetic intervals are never written to the data file.

Here are the rules that Timewarrior follows when handling intervals:

- Ids reference intervals and are assigned after exclusions but before filters are applied. Ids are 1-based array indexes from the end of the list of real + synthetic intervals.
- Intervals can be open or closed.
- There is only one or none open interval.
- There are real and synthetic intervals. Synthetic intervals will always have the lowest id[s].
- The database only contains real closed intervals, and zero or one open interval. An open interval is always the last line in the file.
- Synthetic intervals only exist if an open interval spans over exclusions.
- Exclusions are not applied to closed intervals. Closed intervals may only be modified using a command: move, split, join, modify, shorten, etc. Changing configuration has no effect on closed intervals.
- Synthetic intervals are converted to real intervals if an open interval is closed.
- Cancelling active time tracking removes the open interval, thus removing all synthetic intervals (if any).