FreeBSD date does not feature the -r option GNU date does.

--- man/Makefile.am.orig	2020-10-03 00:15:08 UTC
+++ man/Makefile.am
@@ -54,7 +54,7 @@ $(dist_man_MANS) $(dist_noinst_MANS): REL $(bits)
 REL: $(top_srcdir)/configure
 	@{ echo '.ds Rv $(PACKAGE_VERSION)' ;		\
 	   date -u +'.ds Dt %Y-%m-%d'			\
-	        -r $(top_srcdir)/configure ; } > $@t ;	\
+	        ; } > $@t ;	\
 	if [ -f $@ ] && cmp -s $@t $@ ;			\
 	  then rm -f $@t ;				\
 	  else mv $@t $@ ; echo Created $@ ;		\
--- man/Makefile.in.orig	2020-10-20 04:03:32 UTC
+++ man/Makefile.in
@@ -1736,7 +1736,7 @@ $(dist_man_MANS) $(dist_noinst_MANS): REL $(bits)
 REL: $(top_srcdir)/configure
 	@{ echo '.ds Rv $(PACKAGE_VERSION)' ;		\
 	   date -u +'.ds Dt %Y-%m-%d'			\
-	        -r $(top_srcdir)/configure ; } > $@t ;	\
+	        ; } > $@t ;	\
 	if [ -f $@ ] && cmp -s $@t $@ ;			\
 	  then rm -f $@t ;				\
 	  else mv $@t $@ ; echo Created $@ ;		\