diff --git a/conservancy_beancount/plugin/__init__.py b/conservancy_beancount/plugin/__init__.py
index 3aa9e9c..9183c80 100644
--- a/conservancy_beancount/plugin/__init__.py
+++ b/conservancy_beancount/plugin/__init__.py
@@ -67,6 +67,7 @@ class HookRegistry:
         '.meta_repo_links': None,
         '.meta_rt_links': ['MetaRTLinks'],
         '.meta_tax_implication': None,
+        '.txn_date': ['TransactionDate'],
     }
 
     def __init__(self) -> None:
diff --git a/conservancy_beancount/plugin/txn_date.py b/conservancy_beancount/plugin/txn_date.py
new file mode 100644
index 0000000..dee5e00
--- /dev/null
+++ b/conservancy_beancount/plugin/txn_date.py
@@ -0,0 +1,49 @@
+"""txn_date.py - Validate transactions are entered in the right file by date"""
+# Copyright © 2020  Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+import re
+
+from ..beancount_types import (
+    Transaction,
+)
+
+from . import core
+from .. import config as configmod
+from .. import errors as errormod
+from .. import ranges
+
+class TransactionDate(core.TransactionHook):
+    def __init__(self, config: configmod.Config) -> None:
+        books_path = config.books_path()
+        if books_path is None:
+            raise errormod.ConfigurationError(
+                "books dir setting is required to check transaction dates",
+            )
+        books_pat = re.escape(str(books_path))
+        self.filename_re = re.compile(rf'^{books_pat}/(\d{{4,}})\.beancount$')
+        self.fy = config.fiscal_year_begin()
+
+    def run(self, txn: Transaction) -> errormod.Iter:
+        match = self.filename_re.fullmatch(txn.meta.get('filename', ''))
+        if match is None:
+            return
+        file_fy = int(match.group(1))
+        txn_fy = self.fy.for_date(txn.date)
+        if file_fy != txn_fy:
+            yield errormod.Error(
+                f"transaction dated in FY{txn_fy} entered in FY{file_fy} books",
+                txn,
+            )
diff --git a/setup.py b/setup.py
index e16c8a1..1296504 100755
--- a/setup.py
+++ b/setup.py
@@ -5,7 +5,7 @@ from setuptools import setup
 setup(
     name='conservancy_beancount',
     description="Plugin, library, and reports for reading Conservancy's books",
-    version='1.10.0',
+    version='1.11.0',
     author='Software Freedom Conservancy',
     author_email='info@sfconservancy.org',
     license='GNU AGPLv3+',
diff --git a/tests/test_plugin_txn_date.py b/tests/test_plugin_txn_date.py
new file mode 100644
index 0000000..8c1c056
--- /dev/null
+++ b/tests/test_plugin_txn_date.py
@@ -0,0 +1,79 @@
+"""test_txn_date.py - Unit tests for transaction date validation"""
+# Copyright © 2020  Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+from datetime import date
+
+import pytest
+
+from . import testutil
+
+from conservancy_beancount import config as configmod
+from conservancy_beancount import errors as errormod
+from conservancy_beancount.plugin import txn_date as hookmod
+
+BOOKS_PATH = testutil.test_path('books')
+CONFIG = testutil.TestConfig(books_path=BOOKS_PATH)
+HOOK = hookmod.TransactionDate(CONFIG)
+
+@pytest.mark.parametrize('txn_date,fyear', [
+    (date(2016, 1, 1), 2015),
+    (date(2016, 2, 29), 2015),
+    (date(2016, 3, 1), 2016),
+    (date(2016, 12, 31), 2016),
+    (date(2017, 2, 28), 2016),
+    (date(2017, 3, 1), 2017),
+])
+def test_good_txn(txn_date, fyear):
+    filename = str(BOOKS_PATH / f'{fyear}.beancount')
+    txn = testutil.Transaction(date=txn_date, filename=filename, postings=[
+        ('Assets:Cash', 5),
+        ('Income:Donations', -5),
+    ])
+    assert not list(HOOK.run(txn))
+
+@pytest.mark.parametrize('txn_date,fyear', [
+    (date(2018, 1, 1), 2017),
+    (date(2018, 12, 31), 2018),
+    (date(2019, 3, 1), 2019),
+])
+def test_bad_txn(txn_date, fyear):
+    filename = str(BOOKS_PATH / '2020.beancount')
+    txn = testutil.Transaction(date=txn_date, filename=filename, postings=[
+        ('Assets:Cash', 5),
+        ('Income:Donations', -5),
+    ])
+    errors = list(HOOK.run(txn))
+    assert len(errors) == 1
+    assert errors[0].message == f"transaction dated in FY{fyear} entered in FY2020 books"
+
+@pytest.mark.parametrize('path_s', [
+    'books/2020.beancount',
+    'historical/2020.beancount',
+    'definitions.beancount',
+])
+def test_outer_transactions_not_checked(path_s):
+    txn_date = date(1900, 6, 15)
+    filename = str(BOOKS_PATH / path_s)
+    txn = testutil.Transaction(date=txn_date, filename=filename, postings=[
+        ('Assets:Cash', 5),
+        ('Income:Donations', -5),
+    ])
+    assert not list(HOOK.run(txn))
+
+def test_error_without_books_path():
+    config = configmod.Config()
+    with pytest.raises(errormod.ConfigurationError):
+        hookmod.TransactionDate(config)
diff --git a/tests/testutil.py b/tests/testutil.py
index def9075..298c40e 100644
--- a/tests/testutil.py
+++ b/tests/testutil.py
@@ -255,10 +255,7 @@ class TestConfig:
                  repo_path=None,
                  rt_client=None,
     ):
-        if books_path is None:
-            self._books_loader = None
-        else:
-            self._books_loader = TestBooksLoader(books_path)
+        self._books_path = books_path
         self.fiscal_year = fiscal_year
         self._payment_threshold = Decimal(payment_threshold)
         self.repo_path = test_path(repo_path)
@@ -269,7 +266,13 @@ class TestConfig:
             self._rt_wrapper = rtutil.RT(rt_client)
 
     def books_loader(self):
-        return self._books_loader
+        if self._books_path is None:
+            return None
+        else:
+            return TestBooksLoader(self._books_path)
+
+    def books_path(self):
+        return self._books_path
 
     def books_repo(self):
         return None