timegm: avoid possible failures near start or end of time range.

The previous code could fail for times within the first or last few hours of the representable range, depending on the time zone. Specifically, it failed for times that would be outside the representable range if they were local time, but were representable when they were UTC times due to the time zone offset.

time.asm

@@ -888,6 +888,7 @@ lb1      plb
 timegm   start
          using TimeCommon
 temp     equ   1                        temp variable
+tz_off   equ   5                        time zone offset
 
          csubroutine (4:tmptr),8
          phb
@@ -913,32 +914,28 @@ temp     equ   1                        temp variable
          dey
          lda   [tmptr],Y
          sta   minute
-         lda   [tmptr]
-         sta   second
-         jsr   factor                   compute seconds since 13 Nov 1969 UTC
-         lda   count+4                  if UTC time is unrepresentable
+         jsr   ~get_tz_offset           adjust for time zone offset
+         sta   tz_off
+         stx   tz_off+2
+         clc
+         adc   [tmptr]
+         bcc   lb0
+         inx
+lb0      sta   second
+         stx   second+2
+         jsr   factor_second32          compute seconds since 13 Nov 1969 local
+         lda   count+4                  if time is unrepresentable
          ora   count+6
-         bne   err                        return -1
-lb0      jsr   ~get_tz_offset           convert to local time and save value
-         clc
-         adc   count
-         sta   temp
-         txa
-         adc   count+2
-         sta   temp+2
-         txa                            if local time is unrepresentable
-         bmi   lb0a
-         bcc   lb1
-         clc
-lb0a     bcs   lb1
-err      lda   #-1                        return -1
+         beq   lb1
+         lda   #-1                        return -1
          sta   temp
          sta   temp+2
          bra   lb2
-lb1      ph4   <tmptr                   recompute struct tm values
+lb1      move4 count,temp               save the value for later return
+         ph4   <tmptr                   recompute struct tm values
          pea   0
-         ph4   count
-         ph4   #0
+         ph4   <temp
+         ph4   <tz_off
          jsl   ~gmlocaltime
 lb2      plb
          creturn 4:temp