Fix front matter date location when value gets inherited from other dates

Fixes gohugoio#12236

Author: bep

resources/page/pagemeta/page_frontmatter.go

@@ -421,18 +421,22 @@ func (f *frontmatterFieldHandlers) newDateFieldHandler(key string, setter func(d
 			return false, nil
 		}
 
-		date, err := htime.ToTimeInDefaultLocationE(v, d.Location)
-		if err != nil {
-			return false, nil
+		var date time.Time
+		if vt, ok := v.(time.Time); ok && vt.Location() == d.Location {
+			date = vt
+		} else {
+			var err error
+			date, err = htime.ToTimeInDefaultLocationE(v, d.Location)
+			if err != nil {
+				return false, nil
+			}
+			d.PageConfig.Params[key] = date
 		}
 
 		// We map several date keys to one, so, for example,
 		// "expirydate", "unpublishdate" will all set .ExpiryDate (first found).
 		setter(d, date)
 
-		// This is the params key as set in front matter.
-		d.PageConfig.Params[key] = date
-
 		return true, nil
 	}
 }

resources/page/pagemeta/pagemeta_integration_test.go

@@ -0,0 +1,45 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pagemeta_test
+
+import (
+	"testing"
+
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+func TestLastModEq(t *testing.T) {
+	files := `
+-- hugo.toml --
+timeZone = "Europe/London"
+-- content/p1.md --
+---
+title: p1
+date: 2024-03-13T06:00:00
+---
+-- layouts/_default/single.html --
+Date: {{ .Date }}
+Lastmod: {{ .Lastmod }}
+Eq: {{ eq .Date .Lastmod }}
+
+`
+
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/p1/index.html", `
+Date: 2024-03-13 06:00:00 +0000 GMT
+Lastmod: 2024-03-13 06:00:00 +0000 GMT
+Eq: true
+`)
+}