fix: do formatting in utc to avoid dst gaps (#31)

Fixes #30

Author: andrew0

Diff for: src/__tests__/format.spec.ts

@@ -212,4 +212,14 @@ describe("format with a timezone", () => {
       })
     ).toBe("2024-02-16T12:00:00+0100")
   })
+
+  it("can format times during device DST gaps", () => {
+    expect(
+      format({
+        date: new Date("2024-03-10T02:30:00Z"),
+        format: "HH:mm:ssZ",
+        tz: "UTC",
+      })
+    ).toBe("02:30:00+0000")
+  })
 })

Diff for: src/common.ts

@@ -169,7 +169,7 @@ export function fill(
       return `0${value}`
     }
     if (partName === "dayPeriod") {
-      const p = ap(d.getHours() < 12 ? "am" : "pm", locale)
+      const p = ap(d.getUTCHours() < 12 ? "am" : "pm", locale)
       return token === "A" ? p.toUpperCase() : p.toLowerCase()
     }
     if (partName === "timeZoneName") {
@@ -210,14 +210,17 @@ function createPartMap(
     valueParts.push(
       ...new Intl.DateTimeFormat(
         preciseLocale,
-        requestedParts.reduce((options, part) => {
-          if (part.partName === "literal") return options
-          // Side effect! Genitive parts get shoved into a separate array.
-          if (genitive && genitiveTokens.includes(part.token)) {
-            genitiveParts.push(part)
-          }
-          return Object.assign(options, part.option)
-        }, {} as Intl.DateTimeFormatOptions),
+        requestedParts.reduce(
+          (options, part) => {
+            if (part.partName === "literal") return options
+            // Side effect! Genitive parts get shoved into a separate array.
+            if (genitive && genitiveTokens.includes(part.token)) {
+              genitiveParts.push(part)
+            }
+            return Object.assign(options, part.option)
+          },
+          { timeZone: "UTC" } as Intl.DateTimeFormatOptions
+        )
       )
         .formatToParts(d)
         .map(normStr),
@@ -229,13 +232,15 @@ function createPartMap(
           case "MMMM":
             formattedParts = new Intl.DateTimeFormat(preciseLocale, {
               dateStyle: "long",
+              timeZone: "UTC",
             })
               .formatToParts(d)
               .map(normStr)
             break
           case "MMM":
             formattedParts = new Intl.DateTimeFormat(preciseLocale, {
               dateStyle: "medium",
+              timeZone: "UTC",
             })
               .formatToParts(d)
               .map(normStr)

Diff for: src/format.ts

@@ -5,6 +5,7 @@ import type { DateInput, Format, FormatOptions, Part } from "./types"
 import { offset } from "./offset"
 import { removeOffset } from "./removeOffset"
 import { deviceLocale } from "./deviceLocale"
+import { deviceTZ } from "./deviceTZ"
 
 /**
  * Produce a formatted string. Available strings:
@@ -71,12 +72,16 @@ export function format(
   if (format === "ISO8601") return date(inputDateOrOptions).toISOString()
 
   if (tz) {
-    // If a timezone is provided, we need to apply the offset to the date.
+    forceOffset = offset(inputDateOrOptions, "utc", tz)
+  }
+
+  // We need to apply an offset to the date so that it can be formatted as UTC.
+  tz ??= deviceTZ()
+  if (tz.toLowerCase() !== "utc") {
     inputDateOrOptions = removeOffset(
       inputDateOrOptions,
-      offset(inputDateOrOptions, tz)
+      offset(inputDateOrOptions, tz, "utc")
     )
-    forceOffset = offset(inputDateOrOptions, "utc", tz)
   }
 
   if (!locale || locale === "device") {

Diff for: src/parts.ts

@@ -257,6 +257,7 @@ function partStyle(
         if (style === "long" || style === "short") {
           const genitiveFormattedParts = new Intl.DateTimeFormat(locale, {
             dateStyle: style === "short" ? "medium" : "long",
+            timeZone: "UTC",
           })
             .formatToParts(date)
             .map(normStr)