Calculating Daylight Saving Time

[Heckler]

Hey group,

Been working on code to universally determine if Daylight Saving Time is or is not in effect.
Would love a second set of eyes to make sure I have not missed anything or if there is a cleaner (less code) way of doing it.

I have used the following rules as a basis...

At present, daylight saving time in the United States
begins at 2:00 a.m. on the second Sunday of March and
ends at 2:00 a.m. on the first Sunday of November

below is a shot of a spread sheet that I made to try and account for all possibilities for when a given month (March and November) might begin.
Note 1:the seven colums across represent the seven possibilities for what day of the week the month may begin on.
Note 2: the numbers next to the letter for the day of the week represent the numberd day of the week (ie sun=1, mon=2... sat=7)



Here is my code...

'===========================================
'---  DST Calc (dec1 dow,dec2 mth,dec2 date)  ---
dstcalc:
dst = 1      'covers Apr, May, Jun, Jul, Aug, Sep, Oct
If (mth=1)  then dst = 0  'Jan
If (mth=2)  then dst = 0  'Feb
If (mth=12) then dst = 0  'Dec

March:
if mth=3 then                                       'March
    if  date <8 then
        dst=0
        elseif (date>7 and date<15) and dow=1 and hours<2 then
        dst=0 'Mar
        elseif (date=8  and dow>1) then
        dst=0
        elseif (date=9  and dow>2) then
        dst=0
        elseif (date=10 and dow>3) then
        dst=0
        elseif (date=11 and dow>4) then
        dst=0
        elseif (date=12 and dow>5) then
        dst=0
        elseif (date=13 and dow>6) then
        dst=0
    endif
endif

November:
if  (mth=11) then                                  'November                                      
    if  date >7 then
        dst=0
        elseif (date <8 and dow=1) and hours>1  then
        dst=0               
        elseif (date=2 and dow<3) then
        dst=0
        elseif (date=3 and dow<4) then
        dst=0
        elseif (date=4 and dow<5) then
        dst=0
        elseif (date=5 and dow<6) then
        dst=0
        elseif (date=6 and dow<7) then
        dst=0
    endif
endif
  
return

Note that the code assumes that DST is in effect (dst=1) and tries to find reasons that dst should be set to zero (dst=0)

your thoughts or ways to improve or simplify??

thanks

[pedja089]

I don't understand your tables... Column c,d,e are years? If there are, what years?
I was going to post my code for calculating DST, but in my country winter time is from last Sunday in October to last Sunday in March.
Last Sundays are on same date every 28 years, in fact all days in week are on same day in month every 28 years.
So just build 2 lookup table for 28years. And then use just Year//28 to search for date in that year.
If you look hard you can find link between day in March and day in October.
Here is code for that:

  
    Tmp=Year 'Use less code space than Year//28 in lookup table
    WHILE Tmp>27
        Tmp=Tmp-28
    WEND
    LOOKUP Tmp,[Dates in march for 28 years],Tmp'March
    IF Month=10 THEN  'October
        Tmp=Tmp-4
        IF Tmp<25 THEN Tmp=Tmp+7
    ENDIF

It worked for me, hope that this help.
And using DST flag can complicate life, so I just move hours up or down.

[Heckler]

I don't understand your tables... Column c,d,e are years? If there are, what years?

The seven columns represent ANY year. Since in any given year, the first day of March or November could be a Sun, Mon, Tue, etc. I had to account for that fact that there are 7 POSSIBLE starting days for each month.

The reason I also show Sun=1, Mon=2, etc., is so I could ask "less than" or "greater than" questions.

I wanted this to be a universal calculation for USA.

The code will only apply for these rules...

At present, daylight saving time in the United States
begins at 2:00 a.m. on the second Sunday of March and
ends at 2:00 a.m. on the first Sunday of November

If you are in another part of the world where you have different rules (or no DST at all) then obviously this would not apply.

[MichelJasmin]

Heckler don't you get your time sync from NTP? Do you have the day of week?

If so:

Delta = Date - 7

IF Month=3 AND Day=Sunday AND (Delta>0 OR Delta<8) Then DST!

By the way thank you for the ESPBasic code! Got some ideas...

[enigma]

Hi All
I've used this for many years, works for Europe and I suppose could be adapted for USA. DST Europe changes in March or October, last weekend of the month. The routine calculates the date in the respective month that the change occurs on.
DSTStart holds the start date for DST
DSTEnd holds the end date for DST
Year holds the year in this format e.g 2016

DSTCheck:
DSTStart = (31 - (((5*Year)/4)+ 4) mod 7)
DSTEnd = (31 - (((5*Year/4)) + 1) mod 7)
Return

That's it, Pete

[pedja089]

Can you explain how did you get this formulas?
I was searching whole net few years ago, and best I could think is to type dates for whole century find pattern, and put it in lookup table.
EDIT:
Heckler,
Just found this
http://delphiforfun.org/programs/mat...cs/dstcalc.htm
Basically what Enigma wrote, but it have calculation for US.

[richard]

thanks enigma

that's very neat and tidy
for australia
#au DSTEnd 1st sunday in april = (7- (((5*Year/4)) + 4) % 7)
#au DSTStart 1st sunday in october = (7- (((5*Year/4)) + 5) % 7)