Make GroupByParamDate work with string params

Fixes #3983

resources/page/pagegroup.go

@@ -21,6 +21,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/spf13/cast"
+
 	"github.com/gohugoio/hugo/common/collections"
 	"github.com/gohugoio/hugo/compare"
 
@@ -225,6 +227,10 @@ func (p Pages) groupByDateField(sorter func(p Pages) Pages, formatter func(p Pag
 		sp = sp.Reverse()
 	}
 
+	if sp == nil {
+		return nil, nil
+	}
+
 	date := formatter(sp[0].(Page))
 	var r []PageGroup
 	r = append(r, PageGroup{Key: date, Pages: make(Pages, 0)})
@@ -303,23 +309,36 @@ func (p Pages) GroupByLastmod(format string, order ...string) (PagesGroup, error
 // Valid values for order is asc, desc, rev and reverse.
 // For valid format strings, see https://golang.org/pkg/time/#Time.Format
 func (p Pages) GroupByParamDate(key string, format string, order ...string) (PagesGroup, error) {
-	sorter := func(p Pages) Pages {
+	// Cache the dates.
+	dates := make(map[Page]time.Time)
+
+	sorter := func(pages Pages) Pages {
 		var r Pages
-		for _, e := range p {
-			param := resource.GetParamToLower(e, key)
-			if _, ok := param.(time.Time); ok {
-				r = append(r, e)
+
+		for _, p := range pages {
+			param := resource.GetParamToLower(p, key)
+			var t time.Time
+
+			if param != nil {
+				var ok bool
+				if t, ok = param.(time.Time); !ok {
+					// Probably a string. Try to convert it to time.Time.
+					t = cast.ToTime(param)
+				}
 			}
+
+			dates[p] = t
+			r = append(r, p)
 		}
+
 		pdate := func(p1, p2 Page) bool {
-			p1p, p2p := p1.(Page), p2.(Page)
-			return resource.GetParamToLower(p1p, key).(time.Time).Unix() < resource.GetParamToLower(p2p, key).(time.Time).Unix()
+			return dates[p1].Unix() < dates[p2].Unix()
 		}
 		pageBy(pdate).Sort(r)
 		return r
 	}
 	formatter := func(p Page) string {
-		return resource.GetParamToLower(p, key).(time.Time).Format(format)
+		return dates[p].Format(format)
 	}
 	return p.groupByDateField(sorter, formatter, order...)
 }