Support inexact conference dates

Closes: #188

docs/personal-data-yaml.md

@@ -261,8 +261,25 @@ Required fields:
 - `name`: event name.
 - `topic`: topic/category.
 - `location`: city or location label.
-- `start`: date or datetime.
-- `end`: date or datetime. Must be no earlier than `start`, and duration must be under 20 days.
+- Date information, either as legacy top-level `start` and `end`, or preferred nested `dates`.
+
+Preferred `dates` fields:
+
+- `status`: one of `exact`, `tentative`, or `approximate`.
+- For `exact` and `tentative`: `start` and `end` dates/datetimes. `end` must be no earlier than `start`, and duration must be under 20 days.
+- For `approximate`: `earliest` and `latest` dates for sorting/past-future filtering.
+- `label`: optional human-readable date text. Recommended for `tentative` and `approximate`, for example `likely first weekend of February 2027` or `March 2027`.
+- `basis`: optional explanation of why a tentative date is expected.
+
+Date status behavior:
+
+- `exact`: confirmed dates. These create agenda events, iCalendar entries, and timeline bars.
+- `tentative`: guessed or unconfirmed exact dates. These appear on the conference list with a status badge, but do not create agenda/iCalendar events or timeline bars.
+- `approximate`: only a broad date range is known. These appear on the conference list with a status badge, but do not create agenda/iCalendar events or timeline bars.
+
+Legacy fields:
+
+- Existing top-level `start` and `end` are still supported and are treated as `exact` unless `date_status` says otherwise.
 
 Common optional fields:
 
@@ -273,7 +290,7 @@ Common optional fields:
 - Money/tickets: `free`, `price`, `currency`, `ticket_type`.
 - Other flags: `hackathon`, `attendees`.
 
-Example:
+Exact example:
 
 ```yaml
 - name: FOSDEM
@@ -281,8 +298,10 @@ Example:
   location: Brussels
   country: be
   trip: 2026-02-06
-  start: 2026-02-07
-  end: 2026-02-08
+  dates:
+    status: exact
+    start: 2026-02-07
+    end: 2026-02-08
   attend_start: 2026-02-07 14:00:00+01:00
   attend_end: 2026-02-08
   going: true
@@ -296,6 +315,46 @@ Example:
   longitude: 4.3822
 ```
 
+Tentative example:
+
+```yaml
+- name: FOSDEM
+  topic: FOSDEM
+  location: Brussels
+  country: be
+  dates:
+    status: tentative
+    start: 2027-01-30
+    end: 2027-01-31
+    label: likely first weekend of February 2027
+    basis: FOSDEM is usually on the weekend where Sunday is the first Sunday in February
+  url: https://fosdem.org/2027/
+```
+
+Approximate examples:
+
+```yaml
+- name: Wikimedia Hackathon 2027
+  topic: Wikimedia
+  location: Albania
+  country: al
+  dates:
+    status: approximate
+    label: mid-April 2027
+    earliest: 2027-04-11
+    latest: 2027-04-20
+  hackathon: true
+
+- name: PyCascades 2027
+  topic: Python
+  location: TBC
+  dates:
+    status: approximate
+    label: March 2027
+    earliest: 2027-03-01
+    latest: 2027-03-31
+```
+
 ## `entities.yaml`
 
 Top-level shape: list of people/entities.