Convert Historical Local Time to UTC Time in SQL Server - sql-server

Question

I am confronting an SQL Server database which contains many DateTime values stored in local time. (Yes, this is unfortunate!) We have perhaps 5-10 years of data, which means that the UTC offset for this location will change depending on the time of year if the region in question observes Daylight Savings Time, and of course the schedule on which that change occurred may also change, as for example it did in the United States (where most of these data originate) back in 2007.
My objective is to convert these DateTimes to UTC time at the SQL level. Short of loading the entire Olson or TZ Database and querying it, does anyone have a technique for converting an historical local timestamp to a UTC time? [If it helps, conveniently, we happen to have the latitude and longitude for each row as well (could be used to identify timezone.]
Note: for a row written in real time, the trick of DATEDIFF(Hour, Getutcdate(), GETDATE()) AS UtcOffset works fine, of course. The problem is applying this retroactively to dates that occurred on either side of the Daylight Savings Time "barrier".

Answer 1

You can use AT TIME ZONE to convert to UTC. SQL knows about the switches to daylight savings so it will account for it. You just have to figure out the timezone (using the latitude and longitude, as you said).
You can get all timezones from here:
SELECT * FROM sys.time_zone_info
So the solution will be something like this:
First, add a column to your table with timezone (which you find out using the latitude and longitude).
Then update your (newly added) UTC date column with AT TIME ZONE, for example:
-- some sample data to play with
CREATE TABLE #YourTable
(
LocalDateTime DATETIME,
[UtcDateTime] DATETIMEOFFSET,
TimeZoneName VARCHAR(100)
);
INSERT INTO #YourTable
(
LocalDateTime,
TimeZoneName
)
VALUES
('20150101', 'Alaskan Standard Time'),
('20150101', 'US Mountain Standard Time'),
('20190701', 'Alaskan Standard Time'),
('20190701', 'US Mountain Standard Time');
-- convert to UTC
UPDATE #YourTable
SET [UtcDateTime] = LocalDateTime AT TIME ZONE TimeZoneName AT TIME ZONE 'UTC';
-- check results
SELECT * FROM #YourTable;

Answer 2

This is based on a previous answer by Chris Barlow, at
SQL Server - Convert date field to UTC
This is a solution component in the form of a SQL Server 2008 view that includes a daylight savings (DST) rules approach for historical data conversion.
(No lat/long data needed.)
You can use this view to create your custom solution referencing for update, your local table columns that might need to be converted, like dbo.mytable.created_date.
Some notes on using the view are referenced below, of interest is the section "EXAMPLE USAGE - FOR HISTORICAL DATA CONVERSION":
--
-- DATETIME VS. DATETIMEOFFSET
--
-- WHERE, t = '2016-12-13 04:32:00'
--
declare
#Sydney DATETIME
set
#Sydney = '2016-12-13 04:32:00'
select
Sydney = #Sydney
declare
#Sydney_UTC DATETIMEOFFSET
set
#Sydney_UTC = '2016-12-13 04:32:00.6427663 +10:00'
select
Sydney_UTC = #Sydney_UTC
declare
#NewYork DATETIME
set
#NewYork = '2016-12-13 04:32:00:34'
select
NewYork = #NewYork
declare
#NewYork_UTC DATETIMEOFFSET
set
#NewYork_UTC = '2016-12-13 04:32:00.6427663 -04:00'
select
NewYork_UTC = #NewYork_UTC
select
DATEDIFF(hh, #Sydney, #NewYork) as DIFF_DATETIME
select
DATEDIFF(hh, #Sydney_UTC, #NewYork_UTC) as DIFF_DATETIMEOFFSET
--
-- LOCAL UTC OFFSET FOR REAL-TIME DATA TODAY
--
select
DATEDIFF( Hour, GETUTCDATE(), GETDATE() ) AS UtcOffset
--
-- LOCAL UTC DATE FOR REAL-TIME DATA TODAY - EASTERN STANDARD EXAMPLE
--
select
convert( datetimeoffset( 5 ), GETDATE(), 120 )
--
-- EXAMPLE USAGE -
--
select
*
from
vw_datetime__dst__timezone
--
-- EXAMPLE USAGE - FOR HISTORICAL DATA CONVERSION - EASTERN STANDARD
--
select
created_date,
isnull( dst.zone, 'NO TZ' ) as zone,
isnull(
case
when created_date >= dstlow and
created_date < dsthigh
then dst.daylight
else dst.standard
end,
'NO OFFSET'
) as zone_offsettime,
TODATETIMEOFFSET(
created_date,
case
when created_date >= dstlow and
created_date < dsthigh
then dst.daylight
else dst.standard
end
) as zone_time,
SWITCHOFFSET(
TODATETIMEOFFSET(
created_date,
case
when created_date >= dstlow and
created_date < dsthigh
then dst.daylight
else dst.standard
end
),
'+00:00' -- parameterize?
) as utc_time
from
(
select GETDATE() as created_date
union
select SYSDATETIMEOFFSET() as created_date
union
select '2017-01-01 15:20:24.653' as created_date
) DYNAMIC_temp_table
left outer join vw_datetime__dst__timezone dst on
created_date between yrstart and yrend and
dst.zone = 'ET'
order by
created_date
-- Here is the view SQL:
drop view
vw_datetime__dst__timezone
go
create view
vw_datetime__dst__timezone
as
select
yr,
zone,
standard,
daylight,
rulename,
strule,
edrule,
yrstart,
yrend,
dateadd(day, (stdowref + stweekadd), stmonthref) dstlow,
dateadd(day, (eddowref + edweekadd), edmonthref) dsthigh
from (
select
yrs.yr,
timezone.zone,
timezone.standard,
timezone.daylight,
timezone.rulename,
dst_rule.strule,
dst_rule.edrule,
yrs.yr + '-01-01 00:00:00' yrstart,
yrs.yr + '-12-31 23:59:59' yrend,
yrs.yr + dst_rule.stdtpart + ' ' + dst_rule.cngtime stmonthref,
yrs.yr + dst_rule.eddtpart + ' ' + dst_rule.cngtime edmonthref,
case
when dst_rule.strule in ('1', '2', '3')
then
case
when datepart(dw, yrs.yr + dst_rule.stdtpart) = '1'
then 0
else 8 - datepart(dw, yrs.yr + dst_rule.stdtpart)
end
else (datepart(dw, yrs.yr + dst_rule.stdtpart) - 1) * -1
end as stdowref,
case
when dst_rule.edrule in ('1', '2', '3')
then
case
when datepart(dw, yrs.yr + dst_rule.eddtpart) = '1'
then 0
else 8 - datepart(dw, yrs.yr + dst_rule.eddtpart)
end
else (datepart(dw, yrs.yr + dst_rule.eddtpart) - 1) * -1
end as eddowref,
datename(dw, yrs.yr + dst_rule.stdtpart) as stdow,
datename(dw, yrs.yr + dst_rule.eddtpart) as eddow,
case
when dst_rule.strule in ('1', '2', '3')
then (7 * CAST(dst_rule.strule AS Integer)) - 7
else 0
end as stweekadd,
case
when dst_rule.edrule in ('1', '2', '3')
then (7 * CAST(dst_rule.edrule AS Integer)) - 7
else 0
end as edweekadd
from (
select '1900' yr
union select '1901' yr
union select '1902' yr
union select '1903' yr
union select '1904' yr
union select '1905' yr
union select '1906' yr
union select '1907' yr
union select '1908' yr
union select '1909' yr
union select '1910' yr
union select '1911' yr
union select '1912' yr
union select '1913' yr
union select '1914' yr
union select '1915' yr
union select '1916' yr
union select '1917' yr
union select '1918' yr
union select '1919' yr
union select '1920' yr
union select '1921' yr
union select '1922' yr
union select '1923' yr
union select '1924' yr
union select '1925' yr
union select '1926' yr
union select '1927' yr
union select '1928' yr
union select '1929' yr
union select '1930' yr
union select '1931' yr
union select '1932' yr
union select '1933' yr
union select '1934' yr
union select '1935' yr
union select '1936' yr
union select '1937' yr
union select '1938' yr
union select '1939' yr
union select '1940' yr
union select '1941' yr
union select '1942' yr
union select '1943' yr
union select '1944' yr
union select '1945' yr
union select '1946' yr
union select '1947' yr
union select '1948' yr
union select '1949' yr
union select '1950' yr
union select '1951' yr
union select '1952' yr
union select '1953' yr
union select '1954' yr
union select '1955' yr
union select '1956' yr
union select '1957' yr
union select '1958' yr
union select '1959' yr
union select '1960' yr
union select '1961' yr
union select '1962' yr
union select '1963' yr
union select '1964' yr
union select '1965' yr
union select '1966' yr
union select '1967' yr
union select '1968' yr
union select '1969' yr
union select '1970' yr
union select '1971' yr
union select '1972' yr
union select '1973' yr
union select '1974' yr
union select '1975' yr
union select '1976' yr
union select '1977' yr
union select '1978' yr
union select '1979' yr
union select '1980' yr
union select '1981' yr
union select '1982' yr
union select '1983' yr
union select '1984' yr
union select '1985' yr
union select '1986' yr
union select '1987' yr
union select '1988' yr
union select '1989' yr
union select '1990' yr
union select '1991' yr
union select '1992' yr
union select '1993' yr
union select '1994' yr
union select '1995' yr
union select '1996' yr
union select '1997' yr
union select '1998' yr
union select '1999' yr
union select '2000' yr
union select '2001' yr
union select '2002' yr
union select '2003' yr
union select '2004' yr
union select '2005' yr
union select '2006' yr -- OLD US RULES
union select '2007' yr
union select '2008' yr
union select '2009' yr
union select '2010' yr
union select '2011' yr
union select '2012' yr
union select '2013' yr
union select '2014' yr
union select '2015' yr
union select '2016' yr
union select '2017' yr
union select '2018' yr
union select '2018' yr
union select '2020' yr
union select '2021' yr
union select '2022' yr
union select '2023' yr
union select '2024' yr
union select '2025' yr
union select '2026' yr
union select '2027' yr
union select '2028' yr
union select '2029' yr
union select '2030' yr
union select '2031' yr
union select '2032' yr
union select '2033' yr
union select '2034' yr
union select '2035' yr
union select '2036' yr
union select '2037' yr
union select '2038' yr
union select '2039' yr
union select '2040' yr
union select '2041' yr
union select '2042' yr
union select '2043' yr
union select '2044' yr
union select '2045' yr
union select '2046' yr
union select '2047' yr
union select '2048' yr
union select '2049' yr
union select '2050' yr
union select '2051' yr
union select '2052' yr
union select '2053' yr
union select '2054' yr
union select '2055' yr
union select '2056' yr
union select '2057' yr
union select '2058' yr
union select '2059' yr
union select '2060' yr
union select '2061' yr
union select '2062' yr
union select '2063' yr
union select '2064' yr
union select '2065' yr
union select '2066' yr
union select '2067' yr
union select '2068' yr
union select '2069' yr
union select '2070' yr
union select '2071' yr
union select '2072' yr
union select '2073' yr
union select '2074' yr
union select '2075' yr
union select '2076' yr
union select '2077' yr
union select '2078' yr
union select '2079' yr
union select '2080' yr
union select '2081' yr
union select '2082' yr
union select '2083' yr
union select '2084' yr
union select '2085' yr
union select '2086' yr
union select '2087' yr
union select '2088' yr
union select '2089' yr
union select '2090' yr
union select '2091' yr
union select '2092' yr
union select '2093' yr
union select '2094' yr
union select '2095' yr
union select '2096' yr
union select '2097' yr
union select '2098' yr
union select '2099' yr
) yrs
cross join (
-- Dynamic, hardcoded table of timezone-based, daylight savings time (DST) rules
-- -- TIMEZONE
select 'UTC' zone, '+00:00' standard, '+01:00' daylight, 'UTC' rulename -- UTC - STAGING ONLY - this line is not accurate
union select 'CET' zone, '+01:00' standard, '+02:00' daylight, 'EU' rulename -- Centeral Europe
union select 'ET' zone, '-05:00' standard, '-04:00' daylight, 'US' rulename -- Eastern Time
union select 'CT' zone, '-06:00' standard, '-05:00' daylight, 'US' rulename -- Central Time
union select 'MT' zone, '-07:00' standard, '-06:00' daylight, 'US' rulename -- Mountain Time
union select 'PT' zone, '-08:00' standard, '-07:00' daylight, 'US' rulename -- Pacific Time
) timezone
join (
-- Dynamic, hardcoded table of country-based, daylight savings time (DST) rules
select 'UTC' rulename, 'L' strule, '-03-31' stdtpart, 'L' edrule, '-10-31' eddtpart, 1900 firstyr, 2099 lastyr, '01:00:00' cngtime
-- Country - Europe
union select 'EU' rulename, 'L' strule, '-03-31' stdtpart, 'L' edrule, '-10-31' eddtpart, 1900 firstyr, 2099 lastyr, '01:00:00' cngtime
-- Country - US
union select 'US' rulename, '1' strule, '-04-01' stdtpart, 'L' edrule, '-10-31' eddtpart, 1900 firstyr, 2006 lastyr, '02:00:00' cngtime
union select 'US' rulename, '2' strule, '-03-01' stdtpart, '1' edrule, '-11-01' eddtpart, 2007 firstyr, 2099 lastyr, '02:00:00' cngtime
) dst_rule on
dst_rule.rulename = timezone.rulename and
datepart( year, yrs.yr ) between firstyr and lastyr
) dst_dates
go

Answer 3

I used the following to convert from local Eastern time to UTC (hence the fixed values of 4 and 5 in the function). If you have pre-2007 values, then you would in fact need to modify the udf_IsInDST below to accomodate that as well.
CREATE FUNCTION [dbo].[udf_ConvertTimeLocalToUTC](#dt DATETIME)
RETURNS DATETIME
AS
BEGIN
SET #dt = DATEADD(HOUR, CASE WHEN [dbo].udf_IsInDST(#dt) = 1 THEN 4 ELSE 5 END, #dt)
RETURN #dt
END
GO
CREATE FUNCTION [dbo].[udf_IsInDST](#dt DATETIME)
RETURNS BIT
AS
BEGIN
DECLARE #returnValue BIT = 0
DECLARE #mm INT = DATEPART(MONTH, #dt)
DECLARE #dd INT = DATEPART(DAY, #dt)
DECLARE #dow INT = DATEPART(dw, #dt) -- 1 = sun
DECLARE #hr INT = DATEPART(HOUR, #dt)
SET #returnValue =
CASE WHEN #mm > 3 AND #mm < 11 THEN 1
WHEN #mm = 3 THEN
CASE WHEN #dd < 8 THEN 0
WHEN #dd >= 8 AND #dd <= 14 THEN (CASE WHEN #dow = 1 THEN (CASE WHEN #hr >= 2 THEN 1 ELSE 0 END) ELSE (CASE WHEN #dd - #dow >= 7 THEN 1 ELSE 0 END) END)
ELSE 1
END
WHEN #mm = 11 THEN
CASE WHEN #dd < 7 THEN (CASE WHEN #dow = 1 THEN (CASE WHEN #hr < 2 THEN 1 ELSE 0 END) ELSE (CASE WHEN #dow > #dd THEN 1 ELSE 0 END) END)
ELSE 0
END
ELSE 0
END;
RETURN #returnValue
END
GO

Answer 4

I've used 2 methods in the past.
The first was to create a .Net CLR that takes a datetime and timezone and returns the UTC datetime value which was stored with the data.
The second solution was only required to work for a limited number of time zones and involved creating a table consisting of time zone ID, date from, date to and the correct UTC offset for dates in the past and 20 years in the future. From there it is simple to join and apply the correct offset.