The end of a date range should not be inclusive

If I make an event that spans 1990-1993, and another that spans 1993-1994, I would expect those not to overlap.

However, the current implementation interprets these ranges as:

```
Jan 1, 1990 - Dec 31, 1993 and
Jan 1, 1993 - Dec 31, 1994.
```

Thus they overlap by a year.

Wouldn't it be more appropriate to interpret it as:

```
Jan 1, 1990 - Dec 31, 1992 and
Jan 1, 1993 - Dec 31, 1993.
```

Start by locating the date-range parsing and overlap logic from the timeline entry point. Use the issue's examples as the acceptance criteria: adjacent ranges such as 1990-1993 and 1993-1994 must not overlap, and their displayed end dates must follow the non-inclusive interpretation. No file or test is named, so first trace where ranges are handled and check whether existing coverage can be updated.

Written by the indexing model from the issue text.