diff --git a/pyproject.toml b/pyproject.toml
index b7287b5..867729f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,7 +28,7 @@ numpy = [
     { version = "^2.3", python = ">=3.11" },
 ]
 skyfield-data = "*"
-python-dateutil = "^2.8"
+python-dateutil = ">=2.8,<4.0"
 
 [tool.poetry.group.dev.dependencies]
 black = ">23.1,<25.0"