dim integer gn_UTCHour
 dim integer gn_UTCMin
 dim integer gn_UTCSec
 dim integer gn_UTCDate
 dim integer gn_UTCMonth
 dim integer gn_UTCYear
 dim integer gn_UTCDay
 dim integer gn_LocalHour
 dim integer gn_LocalDate
 dim integer gn_LocalMonth
 dim integer gn_LocalYear
 dim integer gn_LocalDay
 dim integer gn_LocalMins
 dim integer gn_LocalSecs
 
 
 
 ' time conversion
 CONST cSTD_TIME_ZONE = 9.5 ' Adelaide
 CONST CDTS_TIME_ZONE = 10.5
 CONST cSTANDARD_TIME_ZONE = cSTD_TIME_ZONE * 3600 ' + seconds, (i.e South Australia Central Standard Time, +9.5 Hrs from Z time)
 Const cLOCAL_TIME = 1
 Const cZULU_TIME = 2 ' GMT, time zone =0
 

FUNCTION GetDSTStart(ls_Year as String, ln_TimeType as Integer) as integer
' returns time in seconds since 1970 of the actual first second of DST
' DST start occurs at 0200 local on First Sunday in October or 1630Z on Saturday just prior to First Sunday in October
' ls_Year is the year for the calculation in format of "yyyy"
' ln_TimeType determines what the start time will be referenced to, i.e. Zulu (GMT) or local time.
 local as integer ln_Start
 ln_Start = EPOCH("01-10-" + ls_Year + " 02:00:00") + GetOffsetSeconds(ls_Year, "10") ' 2am first sunday of October
 if ln_TimeType = cZULU_TIME then ln_Start = ln_Start - cSTANDARD_TIME_ZONE '1630Z on Sat prior to first Sunday of Oct
 GetDSTStart = ln_Start
end Function

FUNCTION GetDSTEnd(ls_Year as String, ln_TimeType as Integer) as integer
' returns time in seconds since 1970 of the actual first second of Standard Time (non DST period) in UTC (Z or GMT Time)
' DST end occurs at 0300 local on First Sunday in April or 1630 UTC on Saturday just prior to First Sunday in  April
' ls_Year is the year for the calculation in format of "yyyy"
 local as integer ln_End
 ln_End = EPOCH("01-04-" + ls_Year + " 03:00:00") + GetOffsetSeconds(ls_Year, "04") ' 3am fist sunday of April
if ln_TimeType = cZULU_TIME then ln_End = ln_End - cSTANDARD_TIME_ZONE - 3600 '1630Z on Sat prior to first Sunday of month
GetDSTEnd = ln_End
end Function

Function GetOffsetSeconds(ls_Year as String, ls_Month as String) as integer
' returns time in seconds since 1970 of the actual first second of Standard Time (non DST period) in UTC (Z or GMT Time)
'e.g if month starts on a Monday then returns 86400 *6, Tuesday is 86400 * 5, Sunday = 0
 local as integer ln_Offset
 local as String ls_Day
 ls_Day = DAY$("01-" + ls_Month + "-" + ls_Year)
 SELECT CASE ls_Day    
   CASE "Sunday"
     ln_Offset = 0
   CASE "Monday"
     ln_Offset = 1
   CASE "Tuesday"
     ln_Offset = 2
   CASE "Wednesday"
     ln_Offset = 3
   CASE "Thursday"
     ln_Offset = 4
   CASE "Friday"
     ln_Offset = 5
   CASE "Saturday"
     ln_Offset = 6
   CASE ELSE        
     ln_Offset = 0 ' error if here
   END SELECT
   if ln_Offset <> 0 then
     ln_Offset = 7 - ln_Offset
     ln_Offset = ln_Offset * 86400 ' now seconds
   endif
   GetOffsetSeconds = ln_Offset
End Function

SUB StringUTCtoLocal (sYear as string, sDateTime as string)
' sYear as 20xx, sDateTime = dd-mm-yyyy hh:mm:ss
local as integer ln_UTCTime, ln_StartDST, ln_EndDST, ln_LocalTime
ln_StartDST = GetDSTStart(sYear,cZULU_TIME) 'Oct
   ln_EndDST = GetDSTEnd(sYear, cZULU_TIME) 'April
'print "date time = ",sDateTime
ln_UTCTime = EPOCH(sDateTime)
'print "UTC = ", ln_UTCTime, " Start = ",ln_StartDST, "End = ", ln_EndDST
   if ln_UTCTime < ln_StartDST AND ln_UTCTime > ln_EndDST then 'less than oct and more than april
     gf_LocalTimeZone =cSTD_TIME_ZONE
   else
     gf_LocalTimeZone = CDTS_TIME_ZONE
   end if
  ln_LocalTime = ln_UTCTime + gf_LocalTimeZone * 3600
   sDateTime = DATETIME$(ln_LocalTime) ' dd-mm-yyyy hh:mm:ss
  ' print "date Time = ",ls_TempString1
   gn_LocalDate = val(left$(sDateTime,2)) 'dd
gn_LocalMonth = val(mid$(sDateTime,4,2)) 'month
gn_LocalYear = val(mid$(sDateTime,7,4)) 'year, 4 digit
gn_LocalHour = val(mid$(sDateTime,12,2)) 'hour
gn_LocalMins = val(mid$(sDateTime,15,2)) 'min
gn_LocalSecs = val(mid$(sDateTime,18,2))
END SUB

SUB Test
' convert UTC to local
StringUTCtoLocal ("2025", "05-04-2025 16:29:59")
print "UTC Input = ","05-04-2025 16:29:59"
print "LOCAL Out = ",gn_LocalDate, "-", gn_LocalMonth,"-",gn_LocalYear,"   ",gn_LocalHour,":",gn_LocalMins,":",gn_LocalSecs
StringUTCtoLocal ("2025", "05-04-2025 16:30:01")
print "UTC Input = ","05-04-2025 16:30:01"
print "LOCAL Out = ",gn_LocalDate, "-", gn_LocalMonth,"-",gn_LocalYear,"   ",gn_LocalHour,":",gn_LocalMins,":",gn_LocalSecs

StringUTCtoLocal ("2025", "04-10-2025 16:29:59")
print "UTC Input = ","04-10-2025 16:29:59"
print "LOCAL Out = ",gn_LocalDate, "-", gn_LocalMonth,"-",gn_LocalYear,"   ",gn_LocalHour,":",gn_LocalMins,":",gn_LocalSecs
StringUTCtoLocal ("2025", "04-10-2025 16:30:01")
print "UTC Input = ","04-10-2025 16:30:01"
print "LOCAL Out = ",gn_LocalDate, "-", gn_LocalMonth,"-",gn_LocalYear,"   ",gn_LocalHour,":",gn_LocalMins,":",gn_LocalSecs

End SUB

Test

 
 ' as shown, dst starts on the first sunday of April
 ' change the month in the mth1start line to suit and add 7 for second sunday etc.
 ' for last sunday, use the following month and subtract 7 from the dst1
 '
 ' do the same for mth2start and dst2
 ' for the northern hemosphere. reverse the final IF statement
FUNCTION dst()
 LOCAL mth1start, mth2start, dst1, dst2, dateNow$, daynumnow
 dateNow$ = DATE$
 daynumnow = EPOCH(dateNow$+" 00:00:00")/86400
 mth1start = EPOCH("01-04-"+MID$(dateNow$,7,4)+" 00:00:00")/86400
 dst1 = 3-(mth1start MOD 7)
 dst1 = mth1start+dst1 ' first sunday in april in current year. add 7 for 2nd sunday
 
 mth2start = EPOCH("01-10-"+MID$(dateNow$,7,4)+" 00:00:00")/86400
 dst2 = 3-(mth2start MOD 7)
 dst2 = mth2start+ dst2 ' first sunday in october in current year. add 7 for 2nd sunday
 
 IF daynumnow < dst1 OR daynumnow >= dst2 THEN ' swap for northern hemisphere
   dst = 1
 ELSE
   dst = 0
 ENDIF
END FUNCTION
 

IF daynumnow < dst1 OR daynumnow >= dst2 THEN ' swap for northern hemisphere
  dst = 0
ELSE
  dst = 1
ENDIF
END FUNCTION

' TassyJim's amended Daylight Saving function
' If a date (as a string) is specified such as "28/02/2025" it is used, else Date$ is uesd.
' as shown, dst starts on the first sunday of April
' change the month in the mth1start line to suit and add 7 for second sunday etc.
' for last sunday, use the following month and subtract 7 from the dst1
'
' do the same for mth2start and dst2
' for the northern hemosphere. reverse the final IF statement
Function dst(TestDate$)
 Local mth1start, mth2start, dst1, dst2, dateNow$, daynumnow

 If TestDate$ = "" Then
   dateNow$ = Date$
  Else
   dateNow$ = TestDate$
 EndIf

  daynumnow = Epoch(dateNow$+" 00:00:00")/86400
 mth1start = Epoch("01-04-"+Mid$(dateNow$,7,4)+" 00:00:00")/86400
 dst1 = 3-(mth1start Mod 7)
 dst1 = mth1start+dst1 ' first sunday in april in current year. add 7 for 2nd sunday

 mth2start = Epoch("01-10-"+Mid$(dateNow$,7,4)+" 00:00:00")/86400
 dst2 = 3-(mth2start Mod 7)
 dst2 = mth2start+ dst2 ' first sunday in october in current year. add 7 for 2nd sunday

 If daynumnow < dst1 Or daynumnow >= dst2 Then ' swap for northern hemisphere
   dst = 1
 Else
   dst = 0
 EndIf
End Function

> ? dst()
1
> ? dst("07-04-2025")
0
>
> ? not dst()
0
> ? not dst("07-04-2025")
1
>

Function dst(dateNow$)
Local mth1start, mth2start, dst1, dst2, daynumnow

If dateNow$ = "" Then dateNow$ = Date$

> for n=22 to 31:d$=str$(n)+"-03-2025":? d$, dst(d$), day$(d$) :next
22-03-2025       1      Saturday
23-03-2025       1      Sunday
24-03-2025       1      Monday
25-03-2025       1      Tuesday
26-03-2025       1      Wednesday
27-03-2025       1      Thursday
28-03-2025       1      Friday
29-03-2025       1      Saturday
30-03-2025       0      Sunday
31-03-2025       0      Monday
>

FUNCTION dst(TestDate$)
LOCAL mth1start, mth2start, dst1, dst2, dateNow$, daynumnow

IF TestDate$ = "" THEN
  dateNow$ = DATE$
 ELSE
  dateNow$ = TestDate$
ENDIF

 daynumnow = EPOCH(dateNow$+" 00:00:00")/86400
mth1start = EPOCH("01-04-"+MID$(dateNow$,7,4)+" 00:00:00")/86400
dst1 = (10-(mth1start MOD 7)MOD 7)
dst1 = mth1start+dst1 ' first sunday in april in current year. add 7 for 2nd sunday

mth2start = EPOCH("01-10-"+MID$(dateNow$,7,4)+" 00:00:00")/86400
dst2 = (10-(mth2start MOD 7)MOD 7)
dst2 = mth2start+ dst2 ' first sunday in october in current year. add 7 for 2nd sunday

IF daynumnow < dst1 OR daynumnow >= dst2 THEN ' swap for northern hemisphere
  dst = 1
ELSE
  dst = 0
ENDIF
END FUNCTION

Function dst(dateNow$)
 Local mth1start, mth2start, dst1, dst2, daynumnow

 If dateNow$ = "" Then  dateNow$ = Date$

 daynumnow = Epoch(dateNow$+" 00:00:00")/86400
 mth1start = Epoch("01-04-"+Mid$(dateNow$,7,4)+" 00:00:00")/86400
 dst1 = (10-(mth1start Mod 7)Mod 7)
 Inc dst1, mth1start ' first sunday in april in current year. add 7 for 2nd sunday

 mth2start = Epoch("01-10-"+Mid$(dateNow$,7,4)+" 00:00:00")/86400
 dst2 = (10-(mth2start Mod 7)Mod 7)
 Inc dst2, mth2start ' first sunday in october in current year. add 7 for 2nd sunday

 If daynumnow < dst1 Or daynumnow >= dst2 Then ' swap for northern hemisphere
   dst = 1
 Else
   dst = 0
 EndIf
End Function

 '
 DIM startdate$= "01-01-2024"
 DIM startday, tdate$,n,dstchange = 1
 
 startday = EPOCH(startdate$+" 00:00:00")/86400
 FOR n = 0 TO 3660
   tdate$ = LEFT$(DATETIME$((startday+n)*86400),10)
   IF dst(tdate$) <> dstchange THEN
     dstchange = dst(tdate$)
     PRINT tdate$
   ENDIF
 NEXT n
 
FUNCTION dst(TestDate$)
 LOCAL mth1start, mth2start, dst1, dst2, dateNow$, daynumnow
 
 IF TestDate$ = "" THEN
   dateNow$ = DATE$
 ELSE
   dateNow$ = TestDate$
 ENDIF
 
 daynumnow = EPOCH(dateNow$+" 00:00:00")/86400
 mth1start = EPOCH("01-04-"+MID$(dateNow$,7,4)+" 00:00:00")/86400
 dst1 = (10-(mth1start MOD 7))MOD 7
 dst1 = mth1start+dst1 ' first sunday in april in current year. add 7 for 2nd sunday
 
 mth2start = EPOCH("01-10-"+MID$(dateNow$,7,4)+" 00:00:00")/86400
 dst2 = (10-(mth2start MOD 7))MOD 7
 dst2 = mth2start+ dst2 ' first sunday in october in current year. add 7 for 2nd sunday
 
 IF daynumnow < dst1 OR daynumnow >= dst2 THEN ' swap for northern hemisphere
   dst = 1
 ELSE
   dst = 0
 ENDIF
END FUNCTION

startdate$= "01-01-2024"
dstchange = 1

startday = toJD(startdate$)
 FOR n = 0 TO 3660
   tdate$ = datex$(startday+n)
   IF dstx(tdate$) <> dstchange THEN
     dstchange = dstx(tdate$)
     PRINT tdate$
   ENDIF
 NEXT n

FUNCTION dstx(dateNow$) AS INTEGER
 LOCAL INTEGER mth1start, mth2start, dst1, dst2, daynumnow
 IF dateNow$ = "" THEN
   dateNow$ = DATE$
 ENDIF
 daynumnow = toJD(dateNow$)
 mth1start = toJD("01-04-"+MID$(dateNow$,7,4))
 dst1 = (13-(mth1start MOD 7))MOD 7
 dst1 = mth1start+dst1 ' first sunday in april in current year. add 7 for 2nd sunday
 mth2start = toJD("01-10-"+MID$(dateNow$,7,4))
 dst2 = (13-(mth2start MOD 7))MOD 7
 dst2 = mth2start+ dst2 ' first sunday in october in current year. add 7 for 2nd sunday
 IF daynumnow < dst1 OR daynumnow >= dst2 THEN ' swap for northern hemisphere
   dstx = 1
 ELSE
   dstx = 0
 ENDIF
END FUNCTION

FUNCTION toJD(myDate$)' valid for Gregorian dates after Oct 15, 1582
LOCAL d, m, y, ff
d = VAL(LEFT$(myDate$,2))
m = VAL(MID$(myDate$,4,2))
y = VAL(RIGHT$(myDate$,4))
ff=FIX((m-14)/12)
toJD = FIX((1461*(y+4800+ff))/4)+FIX((367*(m-2-12*ff))/12)-FIX((3*(FIX(y+4900+ff)/100))/4)+d-32075
END FUNCTION

FUNCTION datex$(jd)' valid for Gregorian dates after Oct 15, 1582
LOCAL l,n,i,j,d,m,y
l = jd+68569    
n = FIX((4*l)/146097)
l = l-FIX((146097*n+3)/4)
i = FIX((4000*(l+1))/1461001)
l = l-FIX((1461*i)/4)+31
j = FIX((80*l)/2447)
d = l-FIX((2447*j)/80)
l = FIX(j/11)
m = j+2-(12*l)
y = 100*(n-49)+i+l
datex$ = STR$(d,2,0,"0")+"-"+STR$(m,2,0,"0")+"-"+STR$(y,2)
END FUNCTION