diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..98e85d5
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,40 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  Test:
+    runs-on: "${{ matrix.os }}"
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+        python-version:
+          - "3.9"
+          - "3.x"
+    steps:
+      - uses: actions/checkout@v4
+      - name: "Set up Python ${{ matrix.python-version }}"
+        uses: actions/setup-python@v5
+        with:
+          python-version: "${{ matrix.python-version }}"
+          cache: pip
+      - run: "pip install -e .[dev]"
+      - run: py.test -vvv --cov .
+
+  Lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: pre-commit/action@v3.0.1
+
+  Build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: pip install -U build
+      - run: python -m build .
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..7c2a6d6
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,5 @@
+*.py[cod]
+.coverage
+/htmlcov
+build
+dist
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
new file mode 100644
index 0000000..32db09f
--- /dev/null
+++ b/.pre-commit-config.yaml
@@ -0,0 +1,21 @@
+repos:
+  - repo: https://github.com/astral-sh/ruff-pre-commit
+    rev: v0.6.5
+    hooks:
+      - id: ruff
+        args:
+          - --fix
+      - id: ruff-format
+
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.6.0
+    hooks:
+      - id: debug-statements
+      - id: end-of-file-fixer
+      - id: trailing-whitespace
+        exclude: .*ambr
+
+  - repo: https://github.com/pre-commit/mirrors-prettier
+    rev: v3.1.0
+    hooks:
+      - id: prettier
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..107f6df
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2024 Aarni Koskela <akx@iki.fi>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..01a5a50
--- /dev/null
+++ b/README.md
@@ -0,0 +1,67 @@
+# hatch-calver
+
+A plugin for [hatch][hatch] to support [calendar versioning][calver].
+
+## Setup
+
+Add `hatch-calver` as a build dependency to your project.
+
+```toml
+[build-system]
+requires = [
+    "hatchling",
+    "hatch-calver",
+]
+build-backend = "hatchling.build"
+```
+
+Then, set `tool.hatch.version.scheme` to `"calver"`.
+
+```toml
+[tool.hatch.version]
+scheme = "calver"
+```
+
+### Configuring the CalVer scheme
+
+You can optionally set `calver-scheme` to a dot-separated string
+of parts specified in the [calver scheme][calver_scheme] specification.
+It defaults to `YYYY.MM.DD`.
+
+```toml
+[tool.hatch.version]
+scheme = "calver"
+calver-scheme = "YY.MM"
+```
+
+Note that your project's versions should conform to the scheme you specify;
+otherwise, determining where to put e.g. patch versions will be quite ambiguous.
+
+## Usage
+
+You can use Hatch's [standard versioning][hatch_version_updating] commands.
+
+To update your project's version to the current date, run `hatch version release`
+(or `hatch version date`).
+
+As with the regular versioning scheme, you can chain multiple segment updates.
+The date part of the version will _not_ be updated unless you explicitly specify it.
+
+The CalVer scheme specified for your project specifies which segment of the
+PEP 440 "release" segments are automatically determined; for instance, for a `YYYY.MM.DD`
+scheme, the 4th field of the release segment will be considered the `patch` field.
+
+In other words, if you specify `YYYY.MM.DD` as your scheme, and it's the 16th of September 2024:
+
+| Original version | Command                 | New version     |
+|------------------|-------------------------|-----------------|
+| `2024.07.22`     | `hatch version release` | `2024.09.16`    |
+| `2024.07.22`     | `hatch version date,a`  | `2024.09.16a0`  |
+| `2021.01.01`     | `hatch version rc`      | `2021.01.01rc0` |
+| `2024.7.22`      | `hatch version patch`   | `2021.07.22.1`  |
+
+[hatch]: https://hatch.pypa.io/
+[hatch_version_updating]: https://hatch.pypa.io/latest/version/#updating
+[hatch_version_segments]: https://hatch.pypa.io/latest/version/#supported-segments
+[calver]: https://calver.org/
+[calver_scheme]: https://calver.org/#scheme
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..c8a5d4a
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,68 @@
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[project]
+name = "hatch-calver"
+description = "Hatch plugin for CalVer versioning"
+readme = "README.md"
+license = "MIT"
+requires-python = ">=3.9"
+dependencies = ["hatchling"]
+dynamic = ["version"]
+
+authors = [
+    { name = "Aarni Koskela", email = "akx@iki.fi" },
+]
+
+classifiers = [
+    "Development Status :: 4 - Beta",
+    "Framework :: Hatch",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: MIT License",
+    "Natural Language :: English",
+    "Operating System :: MacOS :: MacOS X",
+    "Operating System :: Microsoft :: Windows",
+    "Operating System :: OS Independent",
+    "Operating System :: POSIX :: Linux",
+    "Programming Language :: Python :: 3.9",
+    "Topic :: Software Development",
+]
+
+[project.urls]
+Repository = "https://github.com/akx/hatch-calver"
+Issues = "https://github.com/akx/hatch-calver/issues"
+PyPI = "https://pypi.org/project/hatch-calver/"
+
+[project.entry-points.hatch]
+calver = "hatch_calver.hatch_hooks"
+
+[project.optional-dependencies]
+dev = ["pytest", "pytest-cov"]
+
+[tool.hatch.version]
+path = "src/hatch_calver/__init__.py"
+scheme = "calver"
+calver-scheme = "YYYY.MM.DD"
+
+[tool.coverage.report]
+exclude_lines = [
+    "if __name__ == .__main__.:",
+    "if TYPE_CHECKING:",
+]
+
+[tool.ruff]
+line-length = 110
+
+[tool.ruff.lint]
+select = [
+    "B",
+    "COM812",
+    "E",
+    "F",
+    "I",
+    "W",
+]
+ignore = [
+    "E501",
+]
diff --git a/src/hatch_calver/__init__.py b/src/hatch_calver/__init__.py
new file mode 100644
index 0000000..a08cbd5
--- /dev/null
+++ b/src/hatch_calver/__init__.py
@@ -0,0 +1 @@
+__version__ = "2024.9.16rc0"
diff --git a/src/hatch_calver/bump.py b/src/hatch_calver/bump.py
new file mode 100644
index 0000000..b00aa85
--- /dev/null
+++ b/src/hatch_calver/bump.py
@@ -0,0 +1,119 @@
+"""
+Parts of this code has been adapted from the Hatchling project's
+`hatchling.version.scheme.standard` module, which is licensed under
+the MIT License (Copyright (c) 2017-present Ofek Lev <oss@ofek.dev>).
+"""
+
+from __future__ import annotations
+
+import datetime
+from typing import Any
+
+from packaging.version import Version
+
+SCHEME_PART_TO_FORMATTER = {
+    # See https://calver.org/#scheme
+    "YYYY": lambda d: d.year,
+    "YY": lambda d: d.year - 2000,
+    "0Y": lambda d: f"{d.year % 100:02}",
+    "MM": lambda d: d.month,
+    "0M": lambda d: f"{d.month:02}",
+    "WW": lambda d: d.isocalendar()[1],
+    "0W": lambda d: f"{d.isocalendar()[1]:02}",
+    "DD": lambda d: d.day,
+    "0D": lambda d: f"{d.day:02}",
+}
+
+SCHEME_PART_MIN_LENGTHS = {
+    "YYYY": 4,
+    "YY": 2,
+    "0D": 2,
+    "0M": 2,
+    "0W": 2,
+    "0Y": 2,
+}
+
+
+def _map_scheme_part(part: str, dt: datetime.datetime) -> str:
+    formatter = SCHEME_PART_TO_FORMATTER.get(part)
+    if formatter is None:
+        err = f"Unknown calver-scheme part: {part} (expected one of {', '.join(SCHEME_PART_TO_FORMATTER)})"
+        raise ValueError(err)
+
+    return str(formatter(dt))
+
+
+def _update_version(version: Version, **kwargs: Any) -> None:
+    parts = {}
+    for part_name in ("epoch", "release", "pre", "post", "dev", "local"):
+        if part_name in kwargs:
+            parts[part_name] = kwargs[part_name]
+        elif parts:  # We've set a part, so clear out the following ones
+            parts[part_name] = None
+
+    version._version = version._version._replace(**parts)
+
+
+def bump_calver(
+    original_version: str,
+    desired_version: str,
+    *,
+    calver_scheme_string: str = "YYYY.MM.DD",
+    version_date: datetime.datetime | None = None,
+) -> str:
+    from packaging.version import Version
+    from packaging.version import _parse_letter_version as parse_letter_version
+
+    if not version_date:
+        version_date = datetime.datetime.now(tz=datetime.timezone.utc)
+
+    scheme_parts = str(calver_scheme_string).split(".")
+    v = Version(original_version)
+    instructions = desired_version.split(",")
+    for inst in instructions:
+        if inst in {"date", "release"}:
+            # Update the prefix of the current `release`,
+            # but keep the remaining (non-specified) parts as-is.
+            release = (
+                *(_map_scheme_part(part, version_date) for part in scheme_parts),
+                *v.release[len(scheme_parts) :],
+            )
+            v._version = v._version._replace(release=release)
+            _update_version(v, release=release)
+        elif inst in {"micro", "patch", "fix"}:
+            # We'll assume the first part after any part specified by the calver scheme is the micro/patch/fix part.
+            old_micro = v.release[len(scheme_parts)] if len(v.release) > len(scheme_parts) else 0
+            new_release = (*v.release[: len(scheme_parts)], old_micro + 1)
+            _update_version(v, release=new_release)
+        elif inst in {"a", "b", "c", "rc", "alpha", "beta", "pre", "preview"}:
+            phase, number = parse_letter_version(inst, 0)
+            if v.pre:
+                current_phase, current_number = parse_letter_version(*v.pre)
+                if phase == current_phase:
+                    number = current_number + 1
+
+            _update_version(v, pre=(phase, number))
+        elif inst in {"post", "rev", "r"}:
+            number = 0 if v.post is None else v.post + 1
+            _update_version(v, post=parse_letter_version(inst, number))
+        elif inst == "dev":
+            number = 0 if v.dev is None else v.dev + 1
+            _update_version(v, dev=(inst, number))
+        else:
+            if len(instructions) > 1:
+                raise ValueError("Cannot specify multiple update operations with an explicit version")
+            return str(inst)
+    # Small hack – `packaging.Version` strips leading zeroes from the release part, so
+    # check if we need to pad any of them. This technically breaks the type annotations of
+    # `packaging._Version`, but `Version.__str__()` doesn't mind...
+    v._version = v._version._replace(
+        release=(
+            tuple(
+                str(val).zfill(SCHEME_PART_MIN_LENGTHS.get(scheme_parts[i], 1))
+                if i < len(scheme_parts)
+                else val
+                for (i, val) in enumerate(v._version.release)
+            )
+        ),
+    )
+    return str(v)
diff --git a/src/hatch_calver/calver_scheme.py b/src/hatch_calver/calver_scheme.py
new file mode 100644
index 0000000..16964ee
--- /dev/null
+++ b/src/hatch_calver/calver_scheme.py
@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+from hatchling.version.scheme.plugin.interface import VersionSchemeInterface
+
+from hatch_calver.bump import bump_calver
+
+
+class CalverScheme(VersionSchemeInterface):
+    PLUGIN_NAME = "calver"
+
+    def update(self, desired_version: str, original_version: str, version_data: dict) -> str:
+        if not desired_version:
+            return original_version
+
+        version = bump_calver(
+            original_version,
+            desired_version,
+            calver_scheme_string=self.config.get("calver-scheme", "YYYY.MM.DD"),
+        )
+
+        if self.config.get("validate-bump", True):
+            from packaging.version import Version
+
+            next_version = Version(version)
+            if next_version <= Version(original_version):
+                raise ValueError(
+                    f"Version `{version}` is not higher than the original version `{original_version}`",
+                )
+        return version
diff --git a/src/hatch_calver/hatch_hooks.py b/src/hatch_calver/hatch_hooks.py
new file mode 100644
index 0000000..3bf9a3a
--- /dev/null
+++ b/src/hatch_calver/hatch_hooks.py
@@ -0,0 +1,8 @@
+from hatchling.plugin import hookimpl
+
+
+@hookimpl
+def hatch_register_version_scheme():
+    from hatch_calver.calver_scheme import CalverScheme
+
+    return CalverScheme
diff --git a/tests/test_hatch_calver.py b/tests/test_hatch_calver.py
new file mode 100644
index 0000000..f0be773
--- /dev/null
+++ b/tests/test_hatch_calver.py
@@ -0,0 +1,56 @@
+import datetime
+
+import pytest
+
+from hatch_calver.bump import bump_calver
+
+DT = datetime.datetime(2024, 9, 16, 13, 48)
+
+cases = [
+    # Test that `release`/`date` instructions update the date part to the current date
+    ("0", "release", "2024.9.16", "YYYY.MM.DD"),
+    ("0", "date", "2024.09.16", "YYYY.0M.DD"),
+    ("0.0", "date", "24.38", "YY.WW"),
+    # Test patch
+    ("2024.09.16", "patch", "2024.09.16.1", "YYYY.0M.DD"),
+    ("2024.09.16.1", "patch", "2024.09.16.2", "YYYY.0M.DD"),
+    # Test prereleases
+    ("2024.09.16", "a,alpha", "2024.09.16a1", "YYYY.0M.DD"),
+    ("2024.09.16", "b,beta,b", "2024.09.16b2", "YYYY.0M.DD"),
+    ("2024.09.16", "c,pre,rc", "2024.9.16rc2", "YYYY.M.DD"),  # pre = rc in PEP440 terms
+    # Test postreleases
+    ("2024.09.16", "post,rev,r", "2024.09.16.post2", "YYYY.0M.DD"),
+    # Test dev
+    ("2024.09.16", "dev", "2024.9.16.dev0", "YYYY.M.DD"),
+    # Weird long instructions
+    ("2024.09.16.42", "patch,micro,fix,post,post", "2024.09.16.45.post1", "YYYY.0M.DD"),
+    ("2024.09.10.1.post3", "date,patch,micro,fix,post,post", "24.09.16.4.post1", "YY.0M.DD"),
+    # Cases which do not update the date part (`release`/`date` not specified)
+    ("2023", "patch", "2023.1", "YYYY"),
+    ("2023.12", "patch", "2023.12.1", "YYYY.MM"),
+    # Test that we re-pad packaging.Version-mangled bits
+    ("2024.9.3", "patch", "2024.09.03.1", "YYYY.0M.0D"),
+    # Test simply setting the version
+    ("0", "2024.1.1", "2024.1.1", "this won't matter"),
+]
+
+
+@pytest.mark.parametrize(("original_version", "desired_version", "expected", "scheme"), cases)
+def test_bump(original_version, desired_version, expected, scheme):
+    calver = bump_calver(original_version, desired_version, calver_scheme_string=scheme, version_date=DT)
+    assert calver == expected
+
+
+def test_smoke():
+    # Just exercises the "no date set" path – we can't know what the date will be
+    assert bump_calver("0", "date")
+
+
+def test_bad_spec():
+    with pytest.raises(ValueError):
+        assert bump_calver("0", "date", calver_scheme_string="HEY.THERE")
+
+
+def test_bad_multiple_instructions():
+    with pytest.raises(ValueError):
+        assert bump_calver("0", "2024.8.15,patch")