fix(reminders): show the day for reminders on another day (#46)

A reminder fired ahead of an event on a different day showed only the
event's time (e.g. "09:30 – 10:00"), making it look like it was
happening today. reminderTimeText now prefixes timed events with a
relative day: "Tomorrow"/"Yesterday", the short weekday for another day
this week, or the exact date for anything further out.

The this-week boundary honours the user's "week starts on" setting: the
resolved first day of week is threaded through from ReminderNotifier, so
e.g. a Sunday reads as next week under a Sunday-start locale. All-day
events keep their explicit date (never ambiguous), and cross-midnight
timed events keep both explicit dates.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/data/reminders/ReminderNotifier.kt

@@ -17,7 +17,11 @@ import de.jeanlucmakiola.calendula.R
 import de.jeanlucmakiola.calendula.data.prefs.CalendarPrefs
 import de.jeanlucmakiola.calendula.data.prefs.SettingsPrefs
 import de.jeanlucmakiola.calendula.data.prefs.is24Hour
+import de.jeanlucmakiola.calendula.data.prefs.resolveFirstDay
 import kotlinx.coroutines.flow.first
+import kotlinx.datetime.isoDayNumber
+import java.time.DayOfWeek
+import java.time.Instant
 import java.time.ZoneId
 import java.util.Locale
 import javax.inject.Inject
@@ -55,13 +59,22 @@ class ReminderNotifier @Inject constructor(
         val title = alert.title.ifBlank { context.getString(R.string.event_untitled) }
         val is24Hour = settingsPrefs.timeFormat.first()
             .is24Hour(android.text.format.DateFormat.is24HourFormat(context))
+        val zone = ZoneId.systemDefault()
+        val locale = Locale.getDefault()
+        // resolveFirstDay yields a kotlinx.datetime day; bridge it to java.time by
+        // its shared ISO number (1..7) for the date math in reminderTimeText.
+        val firstDayOfWeek = DayOfWeek.of(settingsPrefs.weekStart.first().resolveFirstDay(locale).isoDayNumber)
         val time = reminderTimeText(
             beginMillis = alert.beginMillis,
             endMillis = alert.endMillis,
             isAllDay = alert.isAllDay,
-            zone = ZoneId.systemDefault(),
-            locale = Locale.getDefault(),
+            zone = zone,
+            locale = locale,
             is24Hour = is24Hour,
+            today = Instant.now().atZone(zone).toLocalDate(),
+            firstDayOfWeek = firstDayOfWeek,
+            tomorrowLabel = context.getString(R.string.reminder_day_tomorrow),
+            yesterdayLabel = context.getString(R.string.reminder_day_yesterday),
         )
         val text = listOfNotNull(time, alert.location).joinToString(" · ")
         val notification = NotificationCompat.Builder(context, CHANNEL_ID)

app/src/main/java/de/jeanlucmakiola/calendula/data/reminders/ReminderTimeText.kt

@@ -1,23 +1,37 @@
 package de.jeanlucmakiola.calendula.data.reminders
 
 import de.jeanlucmakiola.calendula.ui.common.timeOfDayFormatter
+import java.time.DayOfWeek
 import java.time.Instant
+import java.time.LocalDate
 import java.time.ZoneId
 import java.time.ZoneOffset
 import java.time.format.DateTimeFormatter
 import java.time.format.FormatStyle
+import java.time.format.TextStyle
+import java.time.temporal.ChronoUnit
 import java.util.Locale
 
 /**
  * The one line of time context in a reminder notification. Pure so it can be
- * JVM-tested:
+ * JVM-tested.
  *
- * - timed, same day:      "09:30 – 10:00"
- * - timed, crossing days: "11 Jun, 23:30 – 12 Jun, 00:30" (medium date + short time)
+ * Timed events that fall on a day other than [today] are prefixed with that
+ * day, so a reminder fired ahead of time no longer reads as if the event were
+ * today (issue #46). The prefix prefers natural language and stays short:
+ *
+ * - today:                "09:30 – 10:00"                 (no prefix)
+ * - tomorrow / yesterday: "Tomorrow, 09:30 – 10:00"       ([tomorrowLabel] / [yesterdayLabel])
+ * - elsewhere this week:  "Thu, 09:30 – 10:00"            (localized short weekday)
+ * - further out:          "16 Jul, 09:30 – 10:00"         (medium date — a weekday
+ *                                                          alone would be ambiguous)
+ * - timed, crossing days: "11 Jun, 23:30 – 12 Jun, 00:30" (medium date + short time,
+ *                                                          already unambiguous)
  * - all-day, one day:     "11 Jun 2026"
  * - all-day, multi-day:   "11 Jun 2026 – 12 Jun 2026"
  *
- * All-day instances store UTC midnights with an exclusive end, so they are
+ * All-day instances already carry an explicit date, so they never gain a
+ * relative prefix. They store UTC midnights with an exclusive end, so they are
  * read in UTC and the end day is the last *covered* day.
  */
 fun reminderTimeText(
@@ -27,6 +41,10 @@ fun reminderTimeText(
     zone: ZoneId,
     locale: Locale,
     is24Hour: Boolean,
+    today: LocalDate,
+    firstDayOfWeek: DayOfWeek,
+    tomorrowLabel: String,
+    yesterdayLabel: String,
 ): String {
     if (isAllDay) {
         val dateFormat = DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM).withLocale(locale)
@@ -43,18 +61,70 @@ fun reminderTimeText(
     }
 
     val timeFormat = timeOfDayFormatter(is24Hour, locale)
+    val dateFormat = DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM).withLocale(locale)
     val begin = Instant.ofEpochMilli(beginMillis).atZone(zone)
     val end = Instant.ofEpochMilli(endMillis).atZone(zone)
     return if (begin.toLocalDate() == end.toLocalDate()) {
-        timeFormat.format(begin) + RANGE + timeFormat.format(end)
+        val range = timeFormat.format(begin) + RANGE + timeFormat.format(end)
+        val prefix = relativeDayPrefix(
+            day = begin.toLocalDate(),
+            today = today,
+            firstDayOfWeek = firstDayOfWeek,
+            locale = locale,
+            dateFormat = dateFormat,
+            tomorrowLabel = tomorrowLabel,
+            yesterdayLabel = yesterdayLabel,
+        )
+        if (prefix == null) range else "$prefix, $range"
     } else {
         // Cross-day: medium date + the chosen short time, joined per side. Built
         // from the two formatters (not ofLocalizedDateTime) so the 12/24h choice
-        // applies to the time portion too.
-        val dateFormat = DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM).withLocale(locale)
+        // applies to the time portion too. The explicit dates already say which
+        // day, so no relative prefix is layered on top.
         val dateTime = { z: java.time.ZonedDateTime -> "${dateFormat.format(z)}, ${timeFormat.format(z)}" }
         dateTime(begin) + RANGE + dateTime(end)
     }
 }
 
+/**
+ * A short label for [day] relative to [today], or `null` when it *is* today (the
+ * common case, which needs no prefix). Weekday names are used only within the
+ * current week — a "next Wednesday" would be indistinguishable from this one, so
+ * anything past this week falls back to the exact date.
+ */
+private fun relativeDayPrefix(
+    day: LocalDate,
+    today: LocalDate,
+    firstDayOfWeek: DayOfWeek,
+    locale: Locale,
+    dateFormat: DateTimeFormatter,
+    tomorrowLabel: String,
+    yesterdayLabel: String,
+): String? = when (ChronoUnit.DAYS.between(today, day)) {
+    0L -> null
+    1L -> tomorrowLabel
+    -1L -> yesterdayLabel
+    else -> if (isSameWeek(day, today, firstDayOfWeek)) {
+        day.dayOfWeek.getDisplayName(TextStyle.SHORT, locale)
+    } else {
+        dateFormat.format(day)
+    }
+}
+
+/**
+ * True when [day] and [today] share the same week. The week boundary honours the
+ * user's *week starts on* setting (already resolved to a concrete [firstDayOfWeek],
+ * with [firstDayOfWeek] falling back to the locale default upstream).
+ */
+private fun isSameWeek(day: LocalDate, today: LocalDate, firstDayOfWeek: DayOfWeek): Boolean {
+    val startOfWeek = today.previousOrSame(firstDayOfWeek)
+    return !day.isBefore(startOfWeek) && day.isBefore(startOfWeek.plusWeeks(1))
+}
+
+/** The most recent [target] on or before this date (this date itself when it matches). */
+private fun LocalDate.previousOrSame(target: DayOfWeek): LocalDate {
+    val backtrack = (dayOfWeek.value - target.value + 7) % 7
+    return minusDays(backtrack.toLong())
+}
+
 private const val RANGE = " – "

app/src/main/res/values/strings.xml

@@ -226,6 +226,9 @@
     <string name="reminder_onboarding_skip_button">Not now</string>
     <string name="reminder_action_snooze">Snooze</string>
     <string name="reminder_action_dismiss">Dismiss</string>
+    <!-- Day context prefix in a reminder for an event on another day (v2.15.0) -->
+    <string name="reminder_day_tomorrow">Tomorrow</string>
+    <string name="reminder_day_yesterday">Yesterday</string>
 
     <!-- View switcher (M1) -->
     <string name="view_month">Month</string>