Move date converters onto the contracts

Elternbeirat.Contracts/DateOnlyConverter.cs

@@ -0,0 +1,47 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Elternbeirat.Contracts;
+
+/// <summary>
+///     JSON converter that reads and writes a PocketBase date as a calendar date
+///     (<see cref="DateOnly"/>), keeping the day and dropping the time-of-day.
+/// </summary>
+/// <remarks>
+///     PocketBase has no date-only field type: it stores every date as a UTC
+///     instant with a time-of-day (e.g. an editor's <c>2 Jan 00:00</c> Berlin is
+///     stored as <c>"2026-01-01 22:00:00.000Z"</c>). The day the editor meant is
+///     therefore the day <em>in Berlin</em>, not the day of the raw UTC string. This
+///     converter reuses the same UTC-&gt;Berlin conversion as
+///     <see cref="LocalDateTimeConverter"/> and then keeps only the date, so the day
+///     shown is the day that was entered -- with no time-of-day and no off-by-one.
+/// </remarks>
+/// <example>
+///     Apply it to a calendar-day field of a contract:
+/// <code>
+///     [JsonConverter(typeof(DateOnlyConverter))]
+///     public DateOnly Date { get; init; }
+/// </code>
+/// </example>
+/// <seealso cref="WallClock"/>
+public sealed class DateOnlyConverter : JsonConverter<DateOnly>
+{
+    /// <inheritdoc/>
+    /// <exception cref="FormatException">
+    ///     The stored value is not a valid date string.
+    /// </exception>
+    public override DateOnly Read(
+        ref Utf8JsonReader reader,
+        Type typeToConvert,
+        JsonSerializerOptions options)
+        => WallClock.Parse(reader.GetString()) is { } berlin
+            ? DateOnly.FromDateTime(berlin)
+            : default;
+
+    /// <inheritdoc/>
+    public override void Write(
+        Utf8JsonWriter writer,
+        DateOnly value,
+        JsonSerializerOptions options)
+        => writer.WriteStringValue(WallClock.Format(value.ToDateTime(TimeOnly.MinValue)));
+}

Elternbeirat.Contracts/Event.cs

@@ -26,6 +26,7 @@ public record Event
     ///     as the time.
     /// </remarks>
     [JsonPropertyName("start")]
+    [JsonConverter(typeof(LocalDateTimeConverter))]
     public DateTime Start { get; init; }
 
     /// <summary>
@@ -35,6 +36,7 @@ public record Event
     ///     The end time, or <see langword="null"/> if the event has no end.
     /// </value>
     [JsonPropertyName("end")]
+    [JsonConverter(typeof(NullableLocalDateTimeConverter))]
     public DateTime? End { get; init; }
 
     /// <summary>

Elternbeirat.Contracts/LocalDateTimeConverter.cs

@@ -0,0 +1,133 @@
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Elternbeirat.Contracts;
+
+/// <summary>
+///     Converts between PocketBase date strings and Europe/Berlin wall-clock time.
+/// </summary>
+/// <remarks>
+///     PocketBase stores every date in UTC and serializes it with a trailing <c>Z</c>
+///     (e.g. <c>"2026-10-08 17:30:00.000Z"</c>). The values produced here are local
+///     wall-clock numbers: an editor who typed 19:30 in the admin form gets 19:30 back,
+///     with daylight saving applied by the time zone. They carry
+///     <see cref="DateTimeKind.Unspecified"/> so that no later formatting shifts them again.
+/// </remarks>
+internal static class WallClock
+{
+    /// <summary>
+    ///     IANA id; resolves on every platform .NET supports, Windows included.
+    /// </summary>
+    private static readonly TimeZoneInfo Berlin =
+        TimeZoneInfo.FindSystemTimeZoneById("Europe/Berlin");
+
+    /// <summary>
+    ///     Parses a PocketBase UTC date string into Berlin wall-clock time.
+    /// </summary>
+    /// <param name="raw">
+    ///     The raw PocketBase value; may be <see langword="null"/> or empty.
+    /// </param>
+    /// <returns>
+    ///     The Berlin wall-clock time with <see cref="DateTimeKind.Unspecified"/>, or
+    ///     <see langword="null"/> if <paramref name="raw"/> is <see langword="null"/> or empty
+    ///     (PocketBase's representation of an unset date).
+    /// </returns>
+    /// <exception cref="FormatException">
+    ///     <paramref name="raw"/> is not a valid date string.
+    /// </exception>
+    public static DateTime? Parse(string? raw) =>
+        string.IsNullOrEmpty(raw)
+            ? null
+            : TimeZoneInfo.ConvertTime(
+                DateTimeOffset.Parse(raw, CultureInfo.InvariantCulture), Berlin).DateTime;
+
+    /// <summary>
+    ///     Formats a Berlin wall-clock time as the UTC string PocketBase stores.
+    ///     Inverse of <see cref="Parse"/>.
+    /// </summary>
+    /// <param name="berlin">
+    ///     A Berlin wall-clock time. Its <see cref="DateTime.Kind"/> is ignored, and the
+    ///     value is always interpreted as Berlin local time.
+    /// </param>
+    /// <returns>
+    ///     The UTC value in PocketBase format, e.g. <c>"2026-10-08 17:30:00.000Z"</c>.
+    /// </returns>
+    /// <exception cref="ArgumentException">
+    ///     <paramref name="berlin"/> does not exist in Berlin, because it falls in the
+    ///     daylight-saving gap in spring.
+    /// </exception>
+    public static string Format(DateTime berlin) =>
+        TimeZoneInfo.ConvertTimeToUtc(DateTime.SpecifyKind(berlin, DateTimeKind.Unspecified), Berlin)
+            .ToString("yyyy-MM-dd HH:mm:ss.fff'Z'", CultureInfo.InvariantCulture);
+}
+
+/// <summary>
+///     JSON converter that reads and writes PocketBase dates as Europe/Berlin
+///     wall-clock <see cref="DateTime"/> values.
+/// </summary>
+/// <remarks>
+///     An empty string (unset date) is read as <see langword="default"/>(<see cref="DateTime"/>).
+///     For optional dates, use <see cref="NullableLocalDateTimeConverter"/> instead.
+/// </remarks>
+/// <example>
+///     Apply it to a required date field of a contract:
+/// <code>
+///     [JsonConverter(typeof(LocalDateTimeConverter))]
+///     public DateTime Start { get; init; }
+/// </code>
+/// </example>
+/// <seealso cref="WallClock"/>
+public sealed class LocalDateTimeConverter : JsonConverter<DateTime>
+{
+    /// <inheritdoc/>
+    public override DateTime Read(
+        ref Utf8JsonReader reader,
+        Type typeToConvert,
+        JsonSerializerOptions options)
+        => WallClock.Parse(reader.GetString()) ?? default;
+
+    /// <inheritdoc/>
+    public override void Write(
+        Utf8JsonWriter writer,
+        DateTime value,
+        JsonSerializerOptions options)
+        => writer.WriteStringValue(WallClock.Format(value));
+}
+
+/// <summary>
+///     JSON converter that reads and writes optional PocketBase dates as Europe/Berlin
+///     wall-clock <see cref="DateTime"/> values.
+/// </summary>
+/// <remarks>
+///     PocketBase sends an empty string for an unset optional date (e.g. an event
+///     without an end). This value, like JSON <c>null</c>, is read as <see langword="null"/>.
+/// </remarks>
+/// <example>
+///     Apply it to an optional date field of a contract:
+/// <code>
+///     [JsonConverter(typeof(NullableLocalDateTimeConverter))]
+///     public DateTime? End { get; init; }
+/// </code>
+/// </example>
+/// <seealso cref="WallClock"/>
+public sealed class NullableLocalDateTimeConverter : JsonConverter<DateTime?>
+{
+    /// <inheritdoc/>
+    public override DateTime? Read(
+        ref Utf8JsonReader reader,
+        Type typeToConvert,
+        JsonSerializerOptions options)
+        => WallClock.Parse(reader.GetString());
+
+    /// <inheritdoc/>
+    /// <remarks>
+    ///     Only called for non-null values: <see cref="JsonConverter{T}.HandleNull"/> is
+    ///     <see langword="false"/>, so the serializer writes <see langword="null"/> itself.
+    /// </remarks>
+    public override void Write(
+        Utf8JsonWriter writer,
+        DateTime? value,
+        JsonSerializerOptions options)
+        => writer.WriteStringValue(WallClock.Format(value ?? throw new ArgumentNullException(nameof(value))));
+}

Elternbeirat.Contracts/Post.cs

@@ -1,4 +1,3 @@
-using System.ComponentModel;
 using System.Text.Json.Serialization;
 
 namespace Elternbeirat.Contracts;
@@ -27,6 +26,7 @@ public record Post
     ///     time-of-day, no off-by-one at midnight. See <see cref="DateOnlyConverter"/>.
     /// </remarks>
     [JsonPropertyName("date")]
+    [JsonConverter(typeof(DateOnlyConverter))]
     public DateOnly Date { get; init; }
 
     /// <summary>