From 4f09312daf72604f6b7042279ffece8709780818 Mon Sep 17 00:00:00 2001
From: Spencer Brower <6729162+sbrow@users.noreply.github.com>
Date: Fri, 24 Jul 2026 11:22:00 -0400
Subject: [PATCH] feat(mustache): `apply_format` now accepts timezone info.

---
 TODOS.md                 |  3 +-
 mustache/pipes.odin      | 28 +++++++++++++--
 mustache/pipes_test.odin | 73 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 101 insertions(+), 3 deletions(-)

diff --git a/TODOS.md b/TODOS.md
index 9784ee3..ba5ed2a 100644
--- a/TODOS.md
+++ b/TODOS.md
@@ -32,7 +32,8 @@
 ## Dates
 - [x] Accept "strings"
 - [x] Accept keys
-- [ ] handle timezones
+- [x] handle timezones
+- [ ] Fix TZ cache leak in parallel tests: `tz_cache` uses `context.allocator` (tracking allocator in tests), but it's global state shared across parallel test threads. Race causes leak warnings. Fix: use heap allocator for TZ cache.
 - [ ] display an error when no part of the date appears in the output.
 - [x] use `date.format` as the default format.
 - [ ] Handle 0 and whitespace padding i.e. "_2" -> " 2"
diff --git a/mustache/pipes.odin b/mustache/pipes.odin
index c3b3d9f..9d63e28 100644
--- a/mustache/pipes.odin
+++ b/mustache/pipes.odin
@@ -228,7 +228,15 @@ apply_filter :: proc(value: any, filter: ^Pipe_Filter, pos: int, ctx: []any) ->
 			date_format = df
 		}
 
-		str2, err := apply_format(str, filter.args[:], pos, date_format)
+		// Resolve timezone (optional — empty is fine, means display as-is)
+		tz_name := ""
+		if raw := resolve_name("timezone", ctx); raw != nil {
+			if s, s_ok := reflect.as_string(raw); s_ok {
+				tz_name = s
+			}
+		}
+
+		str2, err := apply_format(str, filter.args[:], pos, date_format, tz_name)
 		if err != nil {
 			return value, err
 		} else {
@@ -249,6 +257,7 @@ apply_format :: proc(
 	args: []string,
 	pos: int,
 	date_format: string,
+	timezone_name: string,
 ) -> (
 	result: string,
 	err: Error,
@@ -270,7 +279,22 @@ apply_format :: proc(
 		}
 	}
 
-	log.debugf("date: '%s' format: '%s'", iso, date_format)
+	target_tz, tz_ok := get_cached_tz(timezone_name)
+	if !tz_ok {
+		return "", Error_Body {
+			msg = fmt.tprintf("unable to load timezone '%s'", timezone_name),
+			pos = pos,
+			kind = .Data,
+		}
+	}
+
+	if target_tz != nil {
+		components, _ = convert_to_tz(components, target_tz)
+	} else if components.has_offset {
+		components.tz_abbr = format_offset(components.offset_seconds)
+	}
+
+	log.debugf("date: '%s' format: '%s' tz: '%s'", iso, date_format, timezone_name)
 	return format_date(components, fmt_str), nil
 }
 
diff --git a/mustache/pipes_test.odin b/mustache/pipes_test.odin
index 6a9d622..0a29d9b 100644
--- a/mustache/pipes_test.odin
+++ b/mustache/pipes_test.odin
@@ -511,3 +511,76 @@ test_timezone_empty_when_not_set :: proc(t: ^testing.T) {
 	result, _ := render(tpl, data, {}, context.temp_allocator)
 	testing.expect_value(t, result, "[]")
 }
+
+// ---------------------------------------------------------------------------
+// format pipe with timezone conversion
+// ---------------------------------------------------------------------------
+
+@(test)
+test_format_with_timezone_summer :: proc(t: ^testing.T) {
+	tz, tz_ok := get_cached_tz("America/New_York")
+	defer destroy_tz_cache()
+	if !tz_ok || tz == nil do return
+
+	data := Format_Data {
+		date        = "2026-03-15T12:49:54Z",
+		date_format = "15:04 MST",
+		timezone    = "America/New_York",
+	}
+	tpl, _ := parse("{{date | format}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "08:49 EDT")
+}
+
+@(test)
+test_format_with_timezone_winter :: proc(t: ^testing.T) {
+	tz, tz_ok := get_cached_tz("America/New_York")
+	defer destroy_tz_cache()
+	if !tz_ok || tz == nil do return
+
+	data := Format_Data {
+		date        = "2026-01-15T12:49:54Z",
+		date_format = "15:04 MST",
+		timezone    = "America/New_York",
+	}
+	tpl, _ := parse("{{date | format}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "07:49 EST")
+}
+
+@(test)
+test_format_mst_offset_no_timezone :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date        = "2026-03-15T08:49:54-04:00",
+		date_format = "MST",
+	}
+	tpl, _ := parse("{{date | format}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "UTC-04:00")
+}
+
+@(test)
+test_format_mst_no_offset_no_timezone :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date        = "2026-03-15",
+		date_format = "MST",
+	}
+	tpl, _ := parse("{{date | format}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "UTC")
+}
+
+@(test)
+test_format_invalid_timezone_errors :: proc(t: ^testing.T) {
+	defer destroy_tz_cache()
+
+	data := Format_Data {
+		date        = "2026-03-15",
+		date_format = "2 Jan 2006",
+		timezone    = "Invalid/Zone",
+	}
+	tpl, _ := parse("{{date | format}}", "<test>", allocator = context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "invalid timezone should error")
+}