Here is the code I use to make my timezone table. It's a bit naive, but is usually good enough.
Assumptions:
- It assumes US only rules (DST is 2AM on some pre-defined Sunday,
etc).
- It assumes you don't have dates prior to 1970
- It assumes you know the local timezone offsets (i.e.: EST=-05:00, EDT=-04:00, etc.)
Here's the SQL:
-- make a table (#dst) of years 1970-2101. Note that DST could change in the future and
-- everything was all custom and jacked before 1970 in the US.
declare @first_year varchar(4) = '1970'
declare @last_year varchar(4) = '2101'
-- make a table of all the years desired
if object_id('tempdb..#years') is not null drop table #years
;with cte as (
select cast(@first_year as int) as int_year
,@first_year as str_year
,cast(@first_year + '-01-01' as datetime) as start_of_year
union all
select int_year + 1
,cast(int_year + 1 as varchar(4))
,dateadd(year, 1, start_of_year)
from cte
where int_year + 1 <= @last_year
)
select *
into #years
from cte
option (maxrecursion 500);
-- make a staging table of all the important DST dates each year
if object_id('tempdb..#dst_stage') is not null drop table #dst_stage
select dst_date
,time_period
,int_year
,row_number() over (order by dst_date) as ordinal
into #dst_stage
from (
-- start of year
select y.start_of_year as dst_date
,'start of year' as time_period
,int_year
from #years y
union all
select dateadd(year, 1, y.start_of_year)
,'start of year' as time_period
,int_year
from #years y
where y.str_year = @last_year
-- start of dst
union all
select
case
when y.int_year >= 2007 then
-- second sunday in march
dateadd(day, ((7 - datepart(weekday, y.str_year + '-03-08')) + 1) % 7, y.str_year + '-03-08')
when y.int_year between 1987 and 2006 then
-- first sunday in april
dateadd(day, ((7 - datepart(weekday, y.str_year + '-04-01')) + 1) % 7, y.str_year + '-04-01')
when y.int_year = 1974 then
-- special case
cast('1974-01-06' as datetime)
when y.int_year = 1975 then
-- special case
cast('1975-02-23' as datetime)
else
-- last sunday in april
dateadd(day, ((7 - datepart(weekday, y.str_year + '-04-24')) + 1) % 7, y.str_year + '-04-24')
end
,'start of dst' as time_period
,int_year
from #years y
-- end of dst
union all
select
case
when y.int_year >= 2007 then
-- first sunday in november
dateadd(day, ((7 - datepart(weekday, y.str_year + '-11-01')) + 1) % 7, y.str_year + '-11-01')
else
-- last sunday in october
dateadd(day, ((7 - datepart(weekday, y.str_year + '-10-25')) + 1) % 7, y.str_year + '-10-25')
end
,'end of dst' as time_period
,int_year
from #years y
) y
order by 1
-- assemble a final table
if object_id('tempdb..#dst') is not null drop table #dst
select a.dst_date +
case
when a.time_period = 'start of dst' then ' 03:00'
when a.time_period = 'end of dst' then ' 02:00'
else ' 00:00'
end as start_date
,b.dst_date +
case
when b.time_period = 'start of dst' then ' 02:00'
when b.time_period = 'end of dst' then ' 01:00'
else ' 00:00'
end as end_date
,cast(case when a.time_period = 'start of dst' then 1 else 0 end as bit) as is_dst
,cast(0 as bit) as is_ambiguous
,cast(0 as bit) as is_invalid
into #dst
from #dst_stage a
join #dst_stage b on a.ordinal + 1 = b.ordinal
union all
select a.dst_date + ' 02:00' as start_date
,a.dst_date + ' 03:00' as end_date
,cast(1 as bit) as is_dst
,cast(0 as bit) as is_ambiguous
,cast(1 as bit) as is_invalid
from #dst_stage a
where a.time_period = 'start of dst'
union all
select a.dst_date + ' 01:00' as start_date
,a.dst_date + ' 02:00' as end_date
,cast(0 as bit) as is_dst
,cast(1 as bit) as is_ambiguous
,cast(0 as bit) as is_invalid
from #dst_stage a
where a.time_period = 'end of dst'
order by 1
-------------------------------------------------------------------------------
-- Test Eastern
select
the_date as eastern_local
,todatetimeoffset(the_date, case when b.is_dst = 1 then '-04:00' else '-05:00' end) as eastern_local_tz
,switchoffset(todatetimeoffset(the_date, case when b.is_dst = 1 then '-04:00' else '-05:00' end), '+00:00') as utc_tz
--,b.*
from (
select cast('2015-03-08' as datetime) as the_date
union all select cast('2015-03-08 02:30' as datetime) as the_date
union all select cast('2015-03-08 13:00' as datetime) as the_date
union all select cast('2015-11-01 01:30' as datetime) as the_date
union all select cast('2015-11-01 03:00' as datetime) as the_date
) a left join
#dst b on b.start_date <= a.the_date and a.the_date < b.end_date
