From 6668ec63f7bc136de7973bbf7f6aa876ed6553fe Mon Sep 17 00:00:00 2001
From: Adam Magued <me@adammaged.com>
Date: Wed, 14 Oct 2026 04:01:59 +0000
Subject: [PATCH] feat(rules/en): add timezone parsing rule for common
 timezones and offsets

---
 rules/en/en.go            |  5 +++
 rules/en/timezone.go      | 84 +++++++++++++++++++++++++++++++++++++++
 rules/en/timezone_test.go | 40 +++++++++++++++++++
 3 files changed, 129 insertions(+)
 create mode 100644 rules/en/timezone.go
 create mode 100644 rules/en/timezone_test.go

diff --git a/rules/en/en.go b/rules/en/en.go
index 0b9bd9d..8bbd2ca 100644
--- a/rules/en/en.go
+++ b/rules/en/en.go
@@ -11,6 +11,7 @@ var All = []rules.Rule{
 	Deadline(rules.Override),
 	PastTime(rules.Override),
 	ExactMonthDate(rules.Override),
+	Timezone(rules.Override),
 }
 
 var WEEKDAY_OFFSET = map[string]int{
@@ -167,3 +168,7 @@ var ORDINAL_WORDS = map[string]int{
 }
 
 var ORDINAL_WORDS_PATTERN = `(?:1st|first|2nd|second|3rd|third|4th|fourth|5th|fifth|6th|sixth|7th|seventh|8th|eighth|9th|ninth|10th|tenth|11th|eleventh|12th|twelfth|13th|thirteenth|14th|fourteenth|15th|fifteenth|16th|sixteenth|17th|seventeenth|18th|eighteenth|19th|nineteenth|20th|twentieth|21st|twenty[ -]first|22nd|twenty[ -]second|23rd|twenty[ -]third|24th|twenty[ -]fourth|25th|twenty[ -]fifth|26th|twenty[ -]sixth|27th|twenty[ -]seventh|28th|twenty[ -]eighth|29th|twenty[ -]ninth|30th|thirtieth|31st|thirty[ -]first)`
+
+func init() {
+	// append Timezone rule if needed or include in All
+}
diff --git a/rules/en/timezone.go b/rules/en/timezone.go
new file mode 100644
index 0000000..a9e8827
--- /dev/null
+++ b/rules/en/timezone.go
@@ -0,0 +1,84 @@
+package en
+
+import (
+	"errors"
+	"regexp"
+	"strings"
+	"time"
+
+	"github.com/olebedev/when/rules"
+)
+
+func Timezone(s rules.Strategy) rules.Rule {
+	return &rules.F{
+		RegExp: regexp.MustCompile("(?i)(?:\\W|^)" +
+			"((?:UTC|GMT|EST|EDT|CST|CDT|MST|MDT|PST|PDT|[+-]\\d{2}:?\\d{2}))" +
+			"(?:\\W|$)"),
+		Applier: func(m *rules.Match, c *rules.Context, o *rules.Options, ref time.Time) (bool, error) {
+			if c.Location != nil && s != rules.Override {
+				return false, nil
+			}
+
+			tzStr := m.Captures[0]
+			loc, err := parseTimezone(tzStr)
+			if err != nil {
+				return false, nil
+			}
+
+			c.Location = loc
+			return true, nil
+		},
+	}
+}
+
+func parseTimezone(tz string) (*time.Location, error) {
+	upper := strings.ToUpper(tz)
+	switch upper {
+	case "UTC", "GMT":
+		return time.UTC, nil
+	case "EST":
+		return time.FixedZone("EST", -5*3600), nil
+	case "EDT":
+		return time.FixedZone("EDT", -4*3600), nil
+	case "CST":
+		return time.FixedZone("CST", -6*3600), nil
+	case "CDT":
+		return time.FixedZone("CDT", -5*3600), nil
+	case "MST":
+		return time.FixedZone("MST", -7*3600), nil
+	case "MDT":
+		return time.FixedZone("MDT", -6*3600), nil
+	case "PST":
+		return time.FixedZone("PST", -8*3600), nil
+	case "PDT":
+		return time.FixedZone("PDT", -7*3600), nil
+	}
+
+	loc, err := time.LoadLocation(tz)
+	if err == nil {
+		return loc, nil
+	}
+
+	return parseNumericOffset(tz)
+}
+
+func parseNumericOffset(tz string) (*time.Location, error) {
+	re := regexp.MustCompile(`^([+-])(\d{2}):?(\d{2})?$`)
+	matches := re.FindStringSubmatch(tz)
+	if matches == nil {
+		return nil, errors.New("invalid timezone offset")
+	}
+
+	sign := matches[1]
+	hours, _ := time.ParseDuration(matches[2] + "h")
+	var minutes time.Duration
+	if matches[3] != "" {
+		minutes, _ = time.ParseDuration(matches[3] + "m")
+	}
+	offset := int(hours + minutes)
+	if sign == "-" {
+		offset = -offset
+	}
+
+	return time.FixedZone(tz, offset), nil
+}
diff --git a/rules/en/timezone_test.go b/rules/en/timezone_test.go
new file mode 100644
index 0000000..c39f399
--- /dev/null
+++ b/rules/en/timezone_test.go
@@ -0,0 +1,40 @@
+package en
+
+import (
+	"testing"
+	"time"
+
+	"github.com/olebedev/when/rules"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTimezone(t *testing.T) {
+	rule := Timezone(rules.Override)
+	ref := time.Date(2026, 10, 14, 12, 0, 0, 0, time.UTC)
+
+	tests := []struct {
+		text         string
+		expectedZone string
+		matched      bool
+	}{
+		{"10 PM EST", "EST", true},
+		{"10:30am PDT", "PDT", true},
+		{"tomorrow at 5pm UTC", "UTC", true},
+		{"12:00 +02:00", "+02:00", true},
+		{"invalid", "", false},
+	}
+
+	for _, tc := range tests {
+		m := rule.Find(tc.text)
+		if !tc.matched {
+			assert.Nil(t, m)
+			continue
+		}
+		assert.NotNil(t, m)
+		ctx := &rules.Context{}
+		ok, err := m.Apply(ctx, nil, ref)
+		assert.NoError(t, err)
+		assert.True(t, ok)
+		assert.NotNil(t, ctx.Location)
+	}
+}