Discourage use of version epochs

Add a note discouraging use of epochs in version specifiers, and update
the versioning discussion accordingly.  While at it, suggest using
two-digit years for CalVer, as mentioned in the discussion thread.

Discussion: https://discuss.python.org/t/discouraging-use-of-epoch-segments-in-versions/105811/1
Signed-off-by: Michał Górny <mgorny@quansight.com>

Author: mgorny

source/discussions/versioning.rst

@@ -27,7 +27,7 @@ examples of version numbers [#version-examples]_:
 - A post-release of an alpha release (possible, but discouraged): ``1.2.0a1.post1``
 - A simple version with only two components: ``23.12``
 - A simple version with just one component: ``42``
-- A version with an epoch: ``1!1.0``
+- A version with an epoch (discouraged): ``1!1.0``
 
 Projects can use a cycle of pre-releases to support testing by their users
 before a final release. In order, the steps are: alpha releases, beta releases,
@@ -46,13 +46,14 @@ notes. They should not be used for bug fixes; these should be done with a new
 final release (e.g., incrementing the third component when using semantic
 versioning).
 
-Finally, epochs, a rarely used feature, serve to fix the sorting order when
-changing the versioning scheme. For example, if a project is using calendar
-versioning, with versions like 23.12, and switches to semantic versioning, with
-versions like 1.0, the comparison between 1.0 and 23.12 will go the wrong way.
-To correct this, the new version numbers should have an explicit epoch, as in
-"1!1.0", in order to be treated as more recent than the old version numbers.
-
+Finally, epochs were intended to fix the sorting order when changing the
+versioning scheme. For example, if a project was using calendar versioning, with
+versions like ``23.12``, and switched to semantic versioning, with versions like
+``1.0``, the comparison between ``1.0`` and ``23.12`` would go the wrong way. To
+correct this, the new version numbers would have an explicit epoch, as in
+``1!1.0``, in order to be treated as more recent than the old version numbers.
+However, this is discouraged, and it is preferable to use a higher version
+number that is unlikely to cause user confusion, such as ``100.0``.
 
 
 Semantic versioning vs. calendar versioning

source/specifications/version-specifiers.rst

@@ -394,6 +394,21 @@ from an earlier epoch::
     1!1.1
     1!2.0
 
+.. note::
+
+   Use of nonzero epochs is discouraged. They are often not supported or
+   discouraged by downstream packaging where Python packages may need to be
+   consumed, and due to their scarce use they may also not be well supported by
+   Python packaging tools.
+
+   When version scheme needs to be changed, it is preferable to continue with
+   monotonically increasing numbers in epoch zero. For example, the version
+   2026.x could be unambiguously followed by 3000.x.
+
+   See `Discouraging use of epoch segments in versions
+   <https://discuss.python.org/t/discouraging-use-of-epoch-segments-in-versions/105811>`__
+   for the relevant discussion.
+
 
 .. _version-specifiers-normalization:
 
@@ -1273,3 +1288,4 @@ History
 - May 2025: Clarify that development releases are a form of pre-release when
   they are handled.
 - Nov 2025: Make arbitrary equality case insensitivity explicit.
+- Jan 2026: The use of epochs was discouraged.