UT_LOCAL_TIME — UTC to local time, with daylight saving (function)

The controller clock (CURRENT_DT) is UTC. This gives the time a person on site
reads: the standard offset plus an hour while daylight saving is in force.

Pins

	Pin	Type	Default
in	`Utc`	DT	`DT#1970-01-01-00:00:00`. The clock, e.g. `CURRENT_DT`
in	`OffsetMin`	INT	720. Standard (winter) offset, minutes east of UTC
in	`Dst`	UT_DST_RULE	`UT_DST_NZ`. The daylight saving rule
returns		DT	Local time

Local := UT_LOCAL_TIME(Utc := CURRENT_DT);                                   (* New Zealand *)
Local := UT_LOCAL_TIME(Utc := CURRENT_DT, OffsetMin := 600, Dst := UT_DST_AU_SE);   (* Sydney *)
Summer := UT_DST_ACTIVE(Utc := CURRENT_DT, OffsetMin := 720, Dst := UT_DST_NZ);

Two helper functions come with it:

Function	Returns
`UT_DST_ACTIVE(Utc, OffsetMin, Dst)`	BOOL: daylight saving is in force
`UT_DATE_DAYS(Year, Month, Day)`	DINT: days since 1970-01-01 (a month outside 1..12 rolls into the year before or after)

Rules (`UT_DST_RULE`)

Value	Where	Starts	Ends
`UT_DST_NONE`	no daylight saving
`UT_DST_NZ`	New Zealand	last Sunday in September, 02:00 standard	first Sunday in April, 03:00 daylight
`UT_DST_AU_SE`	NSW, ACT, VIC, TAS, SA	first Sunday in October, 02:00 standard	first Sunday in April, 03:00 daylight
`UT_DST_EU`	European Union, UK	last Sunday in March, 01:00 UTC	last Sunday in October, 01:00 UTC
`UT_DST_US`	United States, Canada	second Sunday in March, 02:00 standard	first Sunday in November, 02:00 daylight

Common offsets: NZ 720, AU east 600, AU central 570, UK 0, central Europe 60,
US Eastern -300, US Pacific -480.

How it works

Local time = Utc + OffsetMin minutes, plus 60 minutes while UT_DST_ACTIVE.
At the change to daylight saving the local clock jumps an hour forward; at the
change back the hour after it repeats. Schedules that use local time see that.
An unknown rule acts as UT_DST_NONE. A result before 1970 gives
DT#1970-01-01-00:00:00. Valid for 1970 to 2099.
Tested against the IANA time zone data a second either side of every change.

OpenPLC

UT_LOCAL_TIME — UTC to local time, with daylight saving (function)

Pins

Rules (UT_DST_RULE)

How it works

Rules (`UT_DST_RULE`)