Generate a POSIX timezone string from a ZoneID (#3514)

core/ui/src/main/kotlin/org/meshtastic/core/ui/component/DropDownPreference.kt

@@ -105,7 +105,7 @@ fun <T> DropDownPreference(
                 enabled = enabled,
                 supportingText =
                 if (summary != null) {
-                    { Text(text = summary, modifier = Modifier.padding(bottom = 8.dp)) }
+                    { Text(text = summary) }
                 } else {
                     null
                 },

core/ui/src/main/kotlin/org/meshtastic/core/ui/component/SwitchPreference.kt

@@ -82,7 +82,7 @@ fun SwitchPreference(
         },
         supportingContent = {
             if (summary.isNotEmpty()) {
-                Text(text = summary, modifier = Modifier.padding(bottom = 16.dp))
+                Text(text = summary)
             }
         },
         headlineContent = { Text(text = title) },

core/ui/src/main/kotlin/org/meshtastic/core/ui/timezone/ZoneIdExtensions.kt

@@ -0,0 +1,134 @@
+/*
+ * Copyright (c) 2025 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+@file:Suppress("Wrapping", "UnusedImports", "SpacingAroundColon")
+
+package org.meshtastic.core.ui.timezone
+
+import java.time.DayOfWeek
+import java.time.Instant
+import java.time.LocalDateTime
+import java.time.ZoneId
+import java.time.ZonedDateTime
+import java.time.format.DateTimeFormatter
+import java.time.temporal.ChronoField
+import java.time.temporal.WeekFields
+import java.util.Locale
+import kotlin.math.abs
+
+/**
+ * Generates a POSIX time zone string from a [ZoneId]. Uses the specification found
+ * [here](https://www.postgresql.org/docs/current/datetime-posix-timezone-specs.html).
+ */
+fun ZoneId.toPosixString(): String {
+    val now = Instant.now()
+    val upcomingTransition = rules.nextTransition(now)
+
+    // No upcoming transition means this time zone does not support DST.
+    if (upcomingTransition == null) {
+        with(now.asZonedDateTime()) {
+            return "${timeZoneShortName()}${formattedOffsetString()}"
+        }
+    }
+
+    val upcomingInstant = upcomingTransition.instant
+    val followingTransition = rules.nextTransition(upcomingInstant)
+
+    val (stdTransition, dstTransition) =
+        if (rules.isDaylightSavings(upcomingInstant)) {
+            followingTransition to upcomingTransition
+        } else {
+            upcomingTransition to followingTransition
+        }
+
+    val stdDate = stdTransition.instant.asZonedDateTime()
+    val dstDate = dstTransition.instant.asZonedDateTime()
+
+    return buildString {
+        append(stdDate.timeZoneShortName())
+        append(stdDate.formattedOffsetString())
+        append(dstDate.timeZoneShortName())
+
+        // Don't append the DST offset if it is only 1 hour off.
+        @Suppress("MagicNumber")
+        if (abs(stdDate.offset.totalSeconds - dstDate.offset.totalSeconds) != 3600) {
+            append(dstDate.formattedOffsetString())
+        }
+
+        append(dstTransition.dateTimeBefore.transitionRuleString())
+        append(stdTransition.dateTimeBefore.transitionRuleString())
+    }
+}
+
+/** Returns the time zone short. e.g. "EST" or "EDT". */
+private fun ZonedDateTime.timeZoneShortName(): String {
+    val formatter = DateTimeFormatter.ofPattern("zzz", Locale.ENGLISH)
+    val shortName = format(formatter)
+    return if (shortName.startsWith("GMT")) "GMT" else shortName
+}
+
+/**
+ * Returns the time zone offset string with the format "<HOURS>:<MINUTES>:<SECONDS>". Minutes and seconds are only shown
+ * if they are non-zero.
+ */
+@Suppress("MagicNumber")
+private fun ZonedDateTime.formattedOffsetString(): String {
+    val offsetSeconds = -offset.totalSeconds
+
+    val hours = offsetSeconds / 3600
+    val remainingSeconds = abs(offsetSeconds) % 3600
+    val minutes = remainingSeconds / 60
+    val seconds = remainingSeconds % 60
+
+    return buildString {
+        append(hours)
+        appendMinSec(minutes = minutes, seconds = seconds) { ":%02d".format(Locale.ENGLISH, it) }
+    }
+}
+
+/**
+ * Returns a transition rule string with the format
+ * ",M<MONTH>.<WEEK_OF_MONTH>.<DAY_OF_WEEK>/<HOURS>:<MINUTES>:<SECONDS>". Time is omitted if it is 2:00:00, since that
+ * is the spec default. Otherwise, append time with non-zero values.
+ */
+@Suppress("MagicNumber")
+private fun LocalDateTime.transitionRuleString() = buildString {
+    val weekOfMonth = get(ChronoField.ALIGNED_WEEK_OF_MONTH)
+    val dayOfWeek = get(WeekFields.of(DayOfWeek.SUNDAY, 7).dayOfWeek()) - 1
+    append(",M$monthValue.$weekOfMonth.$dayOfWeek")
+
+    when {
+        // No-op for spec default
+        hour == 2 && minute == 0 && second == 0 -> Unit
+        else -> {
+            append("/$hour")
+            appendMinSec(minutes = minute, seconds = second) { ":$it" }
+        }
+    }
+}
+
+private inline fun StringBuilder.appendMinSec(minutes: Int, seconds: Int, format: (Int) -> String) {
+    if (minutes != 0 || seconds != 0) {
+        // This covers both "30m:30s" and "00m:30s"
+        append(format(minutes))
+        // This prevents "30m:00s"
+        if (seconds != 0) append(format(seconds))
+    }
+}
+
+context(zoneId: ZoneId)
+private fun Instant.asZonedDateTime() = ZonedDateTime.ofInstant(this, zoneId)

core/ui/src/test/kotlin/org/meshtastic/core/ui/timezone/ZoneIdExtensionsTest.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2025 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.meshtastic.core.ui.timezone
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import java.time.ZoneId
+
+class ZoneIdExtensionsTest {
+
+    @Test
+    fun `test POSIX string generation`() {
+        val zoneMap =
+            mapOf(
+                "US/Hawaii" to "HST10",
+                "US/Alaska" to "AKST9AKDT,M3.2.0,M11.1.0",
+                "US/Pacific" to "PST8PDT,M3.2.0,M11.1.0",
+                "US/Arizona" to "MST7",
+                "US/Mountain" to "MST7MDT,M3.2.0,M11.1.0",
+                "US/Central" to "CST6CDT,M3.2.0,M11.1.0",
+                "US/Eastern" to "EST5EDT,M3.2.0,M11.1.0",
+                "America/Sao_Paulo" to "BRT3",
+                "UTC" to "UTC0",
+                "Europe/London" to "GMT0BST,M3.5.0/1,M10.4.0",
+                "Europe/Lisbon" to "WET0WEST,M3.5.0/1,M10.4.0",
+                "Europe/Budapest" to "CET-1CEST,M3.5.0,M10.4.0/3",
+                "Europe/Kiev" to "EET-2EEST,M3.5.0/3,M10.4.0/4",
+                "Africa/Cairo" to "EET-2EEST,M4.4.5/0,M10.5.5/0",
+                "Asia/Kolkata" to "IST-5:30",
+                "Asia/Hong_Kong" to "HKT-8",
+                "Asia/Tokyo" to "JST-9",
+                "Australia/Perth" to "AWST-8",
+                "Australia/Adelaide" to "ACST-9:30ACDT,M10.1.0,M4.1.0/3",
+                "Australia/Sydney" to "AEST-10AEDT,M10.1.0,M4.1.0/3",
+                "Pacific/Auckland" to "NZST-12NZDT,M9.4.0,M4.1.0/3",
+            )
+
+        zoneMap.forEach { (tz, expected) -> assertEquals(expected, ZoneId.of(tz).toPosixString()) }
+    }
+}