tz algorithm
============

Function: Get UTC offset at a specified UTC time (for converting from UTC to local time)
Input: Requested UTC time (UTC date/time to determine the effective UTC offset for)
Input: Time zone
Output: Base UTC offset and save offset in effect at the requested time
Data: All time zones with their rule sets with their rules, as stored in the tz database text files

Select first rule set of the time zone
Set base UTC offset from the rule set
Set save offset = 0
Set active year = Year(End date of the rule set) - 1
Set date pointer to Jan 1st of the active year

For each rule set of the time zone:
	Set base UTC offset from the rule set
	If the rule set specifies rules:
		Loop with all rules specified in the rule set:
			While the date pointer is in the active year:
				Select the rule with the first transition month/day/time on or after the date pointer from all rules valid in the active year
			|<	Break if no such rule was found
				Compute the UTC transition time from the its relative date, its time type and the active base UTC/save offset
				Recompute the rule set's UTC end time from its time type and the active base UTC/save offset
				If the UTC transition time is greater than or equal to the rule set's UTC end time:
					Set date pointer = rule set's UTC end time
|<	-	-	-	-	Continue with the next rule set
				If the requested UTC time is smaller than the UTC transition time:
					Return with the current base UTC/save offset
				Set save offset from the selected rule
				Set date pointer to the rule's UTC transition time + 1 day
			Increase active year by 1
	If the rule set does not specifiy rules:
		Recompute the rule set's UTC end time from its time type and the active base UTC/save offset
		If the UTC transition time is greater than or equal to the rule set's UTC end time:
			Set date pointer = rule set's UTC end time
|<	-	-	Continue with the next rule set
		If the requested UTC time is smaller than the rule set's UTC end time:
			Return with the current base UTC/save offset