diff --git a/src/apispec/ext/marshmallow/field_converter.py b/src/apispec/ext/marshmallow/field_converter.py
index 0a7b2cf7..79fd368e 100644
--- a/src/apispec/ext/marshmallow/field_converter.py
+++ b/src/apispec/ext/marshmallow/field_converter.py
@@ -549,7 +549,7 @@ def datetime2properties(self, field, **kwargs: typing.Any) -> dict:
                     "example": "1676451245.596",
                     "min": "0",
                 }
-            elif field.format ==  "timestamp_ms":
+            elif field.format == "timestamp_ms":
                 ret = {
                     "type": "number",
                     "format": "float",