The Solaris failure is because their date does not support the '--date'
    or '-r' option. 

As far as I can tell, the date command on Solaris 10 (cfarm210) and 11
(cfarm211) does not support any way to print a specified date value. So
I inserted a fallback to perl -e 'print scalar
gmtime($SOURCE_DATE_EPOCH)' if the date commands fail.

If Perl isn't available either, mdate-sh gives a warning and falls back
to using the mtime of the given file.

    I don't mind just ignorning that platform. 

Not supporting gnulib-tool is one thing, but a lot of people still build
and use Solaris 10. We still get bug reports for Automake from people
running it there. Might as well keep mdate-sh working there, since it
was easy to do.

Below is the important part of the patch, and the new file.
I also threw in a warning if more than one file is specified on the
command line, instead of silently ignoring anything after the first.
Hopefully no one's build actually passes multiple files.

Anyway ... closing this bug, with fingers crossed. This was the last
change I intended to make before the release. Just have to finalize the
Algol 68 naming.  --thanks, karl.

--- a/lib/mdate-sh
+++ b/lib/mdate-sh
@@ -61,12 +66,35 @@ EOF
     ;;
 esac
 
+# Warn if more than one file given.
+if test $# -ne 1; then
+  echo "$0: warning: multiple files given, using first: $*" >&2
+fi
+
 error ()
 {
   echo "$0: $1" >&2
   exit 1
 }
 
+# set $month ("January") and $nummonth (1) given arg MON ("Jan").
+mon_to_month ()
+{
+  case $1 in
+    Jan) month=January; nummonth=1;;
+    Feb) month=February; nummonth=2;;
+    Mar) month=March; nummonth=3;;
+    Apr) month=April; nummonth=4;;
+    May) month=May; nummonth=5;;
+    Jun) month=June; nummonth=6;;
+    Jul) month=July; nummonth=7;;
+    Aug) month=August; nummonth=8;;
+    Sep) month=September; nummonth=9;;
+    Oct) month=October; nummonth=10;;
+    Nov) month=November; nummonth=11;;
+    Dec) month=December; nummonth=12;;
+  esac
+}
 
 # Prevent date giving response in another language.
 LANG=C
@@ -80,6 +108,56 @@ export LC_TIME
 TZ=UTC0
 export TZ
 
+# 
+# https://reproducible-builds.org/docs/source-date-epoch/
+if test -n "$SOURCE_DATE_EPOCH"; then
+  epoch_ok=true # be optimistic
+  date_fmt="+%d %B %Y"
+  result=`date -u --date="@$SOURCE_DATE_EPOCH" "$date_fmt" 2>/dev/null`
+  if test -z "$result"; then
+    result=`date -u -r "$SOURCE_DATE_EPOCH" "$date_fmt" 2>/dev/null`
+    if test -z "$result"; then
+      # The date command on Solaris 10 and 11 doesn't support any way
+      # to do this. Fall back to Perl.
+      #
+      perlout=`perl -e 'print scalar gmtime($SOURCE_DATE_EPOCH)' 2>/dev/null`
+      # Output is, e.g., Thu Jan  1 00:00:00 1970. Split it apart,
+      # since we need to convert "Jan" to "January".
+      # (We could use cut, but surely if a system has perl, it has awk?)
+      day=`echo $perlout | awk '{print $3}'`
+      mon=`echo $perlout | awk '{print $2}'`
+      mon_to_month $mon # sets $month
+      year=`echo $perlout | awk '{print $5}'`
+      result="$day $month $year"
+      #
+      if test -z "$result"; then
+        echo "$0: warning: SOURCE_DATE_EPOCH was set, but can't convert, ignoring: $SOURCE_DATE_EPOCH" >&2
+        epoch_ok=false
+      fi
+    fi
+  fi
+  #
+  if $epoch_ok; then
+    # Remove leading spaces and zeros. We don't want to get into the
+    # various date options to control this. (Not quoting $result here
+    # isn't important, just another way to omit leading spaces.)
+    result=`echo $result | sed 's/^[ 0]*//'`
+    if test -z "$result"; then
+      echo "$0: SOURCE_DATE_EPOCH was set, but converted to empty: $SOURCE_DATE_EPOCH" >&2
+      epoch_ok=false
+    fi
+  fi
+  if $epoch_ok; then
+    echo $result
+    exit 0
+  else
+    echo "$0: SOURCE_DATE_EPOCH failed, falling back to using mtime on: $1" >&2
+  fi
+fi
+# end of SOURCE_DATE_EPOCH support, rest is about the normal case of
+# using the mtime of the specified file.
+
+# 
 # GNU ls changes its time format in response to the TIME_STYLE
 # variable.  Since we cannot assume 'unset' works, revert this
 # variable to its documented default.