The following patch for Xaw3d widget set introduces three additional
resources for the Clock widget:

    hourOffset     Int
    minuteOffset   Int
    gmt            Boolean
    
hourOffset and minuteOffset are somewhat self explanatory.
if gmt is set, gmtime() is used instead of localtime() to
compute tm. if gmt is set to false (the default), the offsets
are relative to localtime, otherwise relative to gmt.

Below is a small wafe script showing the behaviour
================================================================
#! /usr/bin/X11/wafe --f

mergeResources topLevel \
    *background gray85 \
    *borderWidth 0 \
    *Clock.gmt true \
    *Clock.update 1 \
    *Clock.shadowWidth 3

form f topLevel

  clock NY f hourOffset -4
  label NYlabel f fromVert NY label "New York"

  clock Vienna f fromHoriz NY hourOffset +2
  label Vlabel f fromHoriz NY fromVert Vienna label "Vienna"

realize
================================================================

-gustaf
PS: here is the patch:
================================================================
*** Clock.c.orig	Sun Apr 18 15:12:23 1993
--- Clock.c	Sun Apr 18 16:04:59 1993
***************
*** 41,46 ****
--- 41,47 ----
  
  #ifdef X_NOT_STDC_ENV
  extern struct tm *localtime();
+ extern struct tm *gmtime();
  #endif
  
  static void clock_tic(), DrawHand(), DrawSecond(), SetSeg(), DrawClockFace();
***************
*** 95,100 ****
--- 96,107 ----
          offset(font), XtRString, XtDefaultFont},
      {XtNbackingStore, XtCBackingStore, XtRBackingStore, sizeof (int),
      	offset (backing_store), XtRString, "default"},
+     {XtNhourOffset, XtChourOffset, XtRInt, sizeof(int),
+ 	offset(hourOffset), XtRImmediate, (XtPointer) 0},
+     {XtNminuteOffset, XtCminuteOffset, XtRInt, sizeof(int),
+ 	offset(minuteOffset), XtRImmediate, (XtPointer) 0},
+     {XtNgmTime, XtCgmTime, XtRBoolean, sizeof(Boolean),
+ 	offset(gmTime), XtRImmediate, (XtPointer) FALSE},
  };
  
  #undef offset
***************
*** 188,194 ****
         struct tm tm;
         long time_value;
         (void) time(&time_value);
!        tm = *localtime(&time_value);
         str = asctime(&tm);
         if (w->clock.font == NULL)
            w->clock.font = XQueryFont( XtDisplay(w),
--- 195,204 ----
         struct tm tm;
         long time_value;
         (void) time(&time_value);
!        time_value += 
! 	 (long)w->clock.hourOffset*3600 + 
! 	 (long)w->clock.minuteOffset*60;
!        tm = w->clock.gmTime ? *gmtime(&time_value) : *localtime(&time_value);
         str = asctime(&tm);
         if (w->clock.font == NULL)
            w->clock.font = XQueryFont( XtDisplay(w),
***************
*** 329,335 ****
        XtAppAddTimeOut( XtWidgetToApplicationContext( (Widget) w),
  			(unsigned long) w->clock.update*1000, clock_tic, (XtPointer)w );
    (void) time(&time_value);
!   tm = *localtime(&time_value);
    /*
     * Beep on the half hour; double-beep on the hour.
     */
--- 339,348 ----
        XtAppAddTimeOut( XtWidgetToApplicationContext( (Widget) w),
  			(unsigned long) w->clock.update*1000, clock_tic, (XtPointer)w );
    (void) time(&time_value);
!   time_value += 
!     (long)w->clock.hourOffset*3600 + 
!     (long)w->clock.minuteOffset*60;
!   tm = w->clock.gmTime ? *gmtime(&time_value) : *localtime(&time_value);
    /*
     * Beep on the half hour; double-beep on the hour.
     */
*** Clock.h.orig	Sun Apr 18 15:31:27 1993
--- Clock.h	Sun Apr 18 16:15:04 1993
***************
*** 51,60 ****
--- 51,63 ----
   destroyCallback     Callback		Pointer		NULL
   font		     Font		XFontStruct*	fixed
   foreground	     Foreground		Pixel		black
+  gmt	             Gmt	        Boolean         False
   hand		     Foreground		Pixel		black
   height	     Height		Dimension	164
   highlight	     Foreground		Pixel		black
+  hourOffset	     HourOffset	        Int	        0
   mappedWhenManaged   MappedWhenManaged	Boolean		True
+  minuteOffset	     MinuteOffset	Int	        0
   padding	     Margin		int		8
   update		     Interval		int		60 (seconds)
   width		     Width		Dimension	164
***************
*** 77,82 ****
--- 80,93 ----
  
  		/* Int: amount of space around outside of clock */
  #define XtNpadding "padding"
+ 
+ #define XtNhourOffset "hourOffset"
+ #define XtNminuteOffset "minuteOffset"
+ #define XtNgmTime "gmt"
+ 
+ #define XtChourOffset "HourOffset"
+ #define XtCminuteOffset "MinuteOffset"
+ #define XtCgmTime "Gmt"
  
  typedef struct _ClockRec *ClockWidget;  /* completely defined in ClockPrivate.h */
  typedef struct _ClockClassRec *ClockWidgetClass;    /* completely defined in ClockPrivate.h */
*** ClockP.h.orig	Sun Apr 18 15:12:13 1993
--- ClockP.h	Sun Apr 18 16:01:57 1993
***************
*** 56,61 ****
--- 56,64 ----
  	 Boolean beeped;
  	 Boolean analog;
  	 Boolean show_second_hand;
+ 	 Boolean gmTime;
+ 	 int     hourOffset;
+ 	 int     minuteOffset;
  	 Dimension second_hand_length;
  	 Dimension minute_hand_length;
  	 Dimension hour_hand_length;