with t as
(
select getutcdate() dt
)
select dt,
       --dt AT TIME ZONE 'E. Europe Standard Time',
       --cast(dt AT TIME ZONE 'E. Europe Standard Time' as datetime),
       CONVERT(varchar, dt AT TIME ZONE 'E. Europe Standard Time', 103) + ' ' +
       CONVERT(varchar, dt AT TIME ZONE 'E. Europe Standard Time', 108) dt_ukr
  from t      

declare
@p_timezone varchar(100) = 'E. Europe Standard Time';

with t as
(
select getutcdate() dt
)
select dt,
       --dt AT TIME ZONE 'E. Europe Standard Time',
       --cast(dt AT TIME ZONE 'E. Europe Standard Time' as datetime),
       CONVERT(varchar, dt AT TIME ZONE @p_timezone, 103) + ' ' +
       CONVERT(varchar, dt AT TIME ZONE @p_timezone, 108) dt_ukr
  from t      

with tt as
(
select getutcdate() dt, getutcdate() AT TIME ZONE 'E. Europe Standard Time' dt_local
)
select dt,
       dt_local,
       DATEPART(TZOFFSET, dt_local) as DATEPART_TZOFFSET,
       CONVERT(varchar, dt_local, 103) + ' ' +
       CONVERT(varchar, dt_local, 108) dt_ukr,
       dateadd(minute, DATEPART(TZOFFSET, dt_local), dt) dt_ukr_1
  from tt

with t as
(
select {ts '2018-10-28 02:00:00'} dt,1 level
union all
select dateadd(minute, 15, dt), level+1
  from t
 where level <= 6
),
tt as
(
select dt, dt AT TIME ZONE 'E. Europe Standard Time' dt_local
  from t
)
select dt,
       dt_local,
       DATEPART(TZOFFSET, dt_local) as DATEPART_TZOFFSET,
       CONVERT(varchar, dt_local, 103) + ' ' +
       CONVERT(varchar, dt_local, 108) dt_ukr,
       dateadd(minute, DATEPART(TZOFFSET, dt_local), dt) dt_ukr_1
  from tt
;

with tt as
(
select getutcdate() dt
)
select
 tt.dt, cast(switchoffset(tt.dt, tzi.current_utc_offset) as datetime)
from
 tt join
 sys.time_zone_info tzi on tzi.name = N'E. Europe Standard Time';

with t as
(
select {ts '2018-10-28 02:00:00'} dt,1 level
union all
select dateadd(minute, 30, dt), level+1
  from t
 where level <= 10
),
tt as
(
select dt, dt AT TIME ZONE 'FLE Standard Time' dt_local
  from t
)
select dt,
       dt_local,
       DATEPART(TZOFFSET, dt_local) as DATEPART_TZOFFSET,
       CONVERT(varchar, dt_local, 103) + ' ' +
       CONVERT(varchar, dt_local, 108) dt_ukr,
       dateadd(minute, DATEPART(TZOFFSET, dt_local), dt) dt_ukr_1
  from tt
;

            DateTime utcDt = DateTime.Now.ToUniversalTime();
            utcDt = DateTime.Parse("2018-10-28 00:00");
            int i;
            string msg = "";
            for (i = 1; i < 11; i++)
            {
                utcDt = utcDt.AddMinutes(15);
                msg += utcDt.ToString() + " / " +
                       utcDt.ToLocalTime().ToString() + " / " +
                       (utcDt - utcDt.ToLocalTime()).ToString() +
                       Environment.NewLine;
            }
            MessageBox.Show(msg);

    DateTime utcDt;
    TimeZoneInfo cstZone = TimeZoneInfo.FindSystemTimeZoneById("FLE Standard Time");
    MessageBox.Show(cstZone.ToString());
    utcDt = DateTime.Parse("2018-10-28 00:00");
    int i;
    string msg = "";
    for (i = 1; i < 11; i++)
    {
        utcDt = utcDt.AddMinutes(15);
        msg += utcDt.ToString() + " / " +
                utcDt.ToLocalTime().ToString() + " / " +
                TimeZoneInfo.ConvertTimeFromUtc(utcDt, cstZone) + "/" +
                (utcDt - utcDt.ToLocalTime()).ToString() +
                Environment.NewLine;
    }
    MessageBox.Show(msg);

28.10.2018 0:15:00 / 28.10.2018 3:15:00 / 28.10.2018 3:15:00/-03:00:00
28.10.2018 0:30:00 / 28.10.2018 3:30:00 / 28.10.2018 3:30:00/-03:00:00
28.10.2018 0:45:00 / 28.10.2018 3:45:00 / 28.10.2018 3:45:00/-03:00:00
28.10.2018 1:00:00 / 28.10.2018 3:00:00 / 28.10.2018 3:00:00/-02:00:00
28.10.2018 1:15:00 / 28.10.2018 3:15:00 / 28.10.2018 3:15:00/-02:00:00
28.10.2018 1:30:00 / 28.10.2018 3:30:00 / 28.10.2018 3:30:00/-02:00:00
28.10.2018 1:45:00 / 28.10.2018 3:45:00 / 28.10.2018 3:45:00/-02:00:00
28.10.2018 2:00:00 / 28.10.2018 4:00:00 / 28.10.2018 4:00:00/-02:00:00
28.10.2018 2:15:00 / 28.10.2018 4:15:00 / 28.10.2018 4:15:00/-02:00:00
28.10.2018 2:30:00 / 28.10.2018 4:30:00 / 28.10.2018 4:30:00/-02:00:00

alter FUNCTION [dbo].ToTimeZoneDateTime
(
    @p_dt [datetime],
	@p_TIME_ZONE_name varchar(100) = 'FLE Standard Time'
)
RETURNS datetime
AS
BEGIN
    declare @v_dt   datetimeoffset(3);

    set @v_dt = @p_dt at time zone @p_TIME_ZONE_name;

    RETURN dateadd(minute, DATEPART(TZOFFSET, @v_dt), @p_dt);

END

select getutcdate() getutcdate, [dbo].ToTimeZoneDateTime(getutcdate(), default) ToTimeZoneDateTime, getdate() getdate;

alter FUNCTION [dbo].ChangeTimeZoneDateTime
(
    @p_dt [datetime],
	@p_from_TIME_ZONE_name varchar(100),
	@p_to_TIME_ZONE_name varchar(100)
)
RETURNS datetime
AS
BEGIN
    return @p_dt at time zone @p_from_TIME_ZONE_name AT TIME ZONE @p_to_TIME_ZONE_name;
END

with t as
(
select {ts '2018-10-27 23:00:00'} dt,1 level
union all
select dateadd(minute, 30, dt), level+1
  from t
 where level <= 14
)
select dt,
       [dbo].ChangeTimeZoneDateTime(dt, 'UTC', 'FLE Standard Time') ChangeTimeZoneDateTime_from_UTC_TO_FLE,
       [dbo].ChangeTimeZoneDateTime(dt, 'FLE Standard Time', 'UTC') ChangeTimeZoneDateTime_from_FLE_TO_UTC
  from t