fvwm

th@debian:~$ cat /usr/share/fvwm/FvwmScript-DateTime
WindowTitle {DateTime}
WindowSize  120 35
Font "xft:Sans:style=Bold:size=10" 

Init
 Begin
  Set $Date = (GetOutput {exec date "+%a  %d-%b-%Y  %H:%M:%S"} 1 -1)

  ChangeTitle 2 $Date
End

PeriodicTasks
 Begin 
  If (RemainderOfDiv (GetTime) 15)==0 Then
  Begin
   Set $Date = (GetOutput {exec date "+%a  %d-%b-%Y  %H:%M:%S"} 1 -1)

   ChangeTitle 2 $Date
  End
End

Widget 1
Property
 Position 0 0
 Size 120 20
 Font "xft:Sans:style=Bold:size=11"
 Type ItemDraw
 Flags NoReliefString
 Title {}
Main
 Case message of
  SingleClic :
  Begin
  End
End

Widget 2
Property
 Position 0 20
 Size 120 15
 Font "xft:Sans:style=Bold:size=8"
 Type ItemDraw
 Flags NoReliefString
 Title {}
Main
 Case message of
  SingleClic :
  Begin
  End
End
th@debian:~$ 

th@debian:~$ fvwm -V
fvwm 2.6.8 compiled on Jun  3 2018 at 16:51:00
with support for: ReadLine, RPlay, Stroke, XPM, SVG, Shape, XShm, SM, Bidi text, Xinerama, XRender, XCursor, XFT, NLS

fvwm comes with NO WARRANTY, to the extent permitted by law. You may
redistribute copies of fvwm under the terms of the GNU General Public License.
For more information about these matters, see the file named COPYING.
th@debian:~$