[Issue #2] - Added timezone support with a list of support timezones

time.go

@@ -0,0 +1,91 @@
+package main
+
+import (
+	"time"
+)
+
+type TimezoneConfig struct {
+	StandardName   string
+	StandardOffset int
+	DaylightName   string
+	DaylightOffset int
+	ObservesUSDST  bool
+}
+
+// Map timezone aliases to standard/daylight offsets
+var timezones = map[string]TimezoneConfig{
+	// Dynamic US zones (switch automatically between Standard and Daylight)
+	"US/Eastern":  {"EST", -5 * 3600, "EDT", -4 * 3600, true},
+	"ET":          {"EST", -5 * 3600, "EDT", -4 * 3600, true},
+	"US/Central":  {"CST", -6 * 3600, "CDT", -5 * 3600, true},
+	"CT":          {"CST", -6 * 3600, "CDT", -5 * 3600, true},
+	"US/Mountain": {"MST", -7 * 3600, "MDT", -6 * 3600, true},
+	"MT":          {"MST", -7 * 3600, "MDT", -6 * 3600, true},
+	"US/Pacific":  {"PST", -8 * 3600, "PDT", -7 * 3600, true},
+	"PT":          {"PST", -8 * 3600, "PDT", -7 * 3600, true},
+
+	// Fixed non-DST zones
+	"UTC": {"UTC", 0, "UTC", 0, false},
+	"EST": {"EST", -5 * 3600, "EST", -5 * 3600, false},
+	"EDT": {"EDT", -4 * 3600, "EDT", -4 * 3600, false},
+	"CST": {"CST", -6 * 3600, "CST", -6 * 3600, false},
+	"CDT": {"CDT", -5 * 3600, "CDT", -5 * 3600, false},
+	"MST": {"MST", -7 * 3600, "MST", -7 * 3600, false},
+	"MDT": {"MDT", -6 * 3600, "MDT", -6 * 3600, false},
+	"PST": {"PST", -8 * 3600, "PST", -8 * 3600, false},
+	"PDT": {"PDT", -7 * 3600, "PDT", -7 * 3600, false},
+}
+
+// IsUSDST checks if the given UTC time falls in US Daylight Saving Time
+// (Second Sunday in March at 02:00 local to first Sunday in November at 02:00 local).
+func IsUSDST(t time.Time, stdOffsetSec int) bool {
+	month := t.Month()
+	if month < time.March || month > time.November {
+		return false // Dec, Jan, Feb
+	}
+	if month > time.March && month < time.November {
+		return true // Apr through Oct
+	}
+
+	day := t.Day()
+	weekday := int(t.Weekday())
+
+	if month == time.March {
+		// Second Sunday in March is between 8th and 14th
+		prevSunday := day - weekday
+		if prevSunday < 8 {
+			return false
+		}
+		if prevSunday >= 15 {
+			return true
+		}
+		// Transition occurs at 2:00 AM standard local time -> (2 - stdOffsetHours) in UTC
+		stdOffsetHours := stdOffsetSec / 3600
+		switchHourUTC := 2 - stdOffsetHours
+		return weekday != int(time.Sunday) || t.Hour() >= switchHourUTC
+	}
+
+	// November: first Sunday is between 1st and 7th
+	prevSunday := day - weekday
+	if prevSunday < 1 {
+		return true // Before first Sunday
+	}
+	// Fall-back occurs at 2:00 AM daylight local time -> (2 - (stdOffsetHours + 1)) in UTC
+	stdOffsetHours := stdOffsetSec / 3600
+	switchHourUTC := 2 - (stdOffsetHours + 1)
+	return weekday == int(time.Sunday) && t.Hour() < switchHourUTC
+}
+
+// ResolveTimezone dynamically determines the *time.Location and label
+// based on whether the zone observes DST at the given UTC time.
+func ResolveTimezone(name string, utcNow time.Time) (*time.Location, string) {
+	cfg, ok := timezones[name]
+	if !ok {
+		return time.FixedZone("UTC", 0), "UTC"
+	}
+
+	if cfg.ObservesUSDST && IsUSDST(utcNow, cfg.StandardOffset) {
+		return time.FixedZone(cfg.DaylightName, cfg.DaylightOffset), cfg.DaylightName
+	}
+	return time.FixedZone(cfg.StandardName, cfg.StandardOffset), cfg.StandardName
+}