Remote posts take their timeline place on arrival, and an unedited one has no edit date

Found by the first live run against GoToSocial:

- A remote post's id came from its published second plus random bytes, so a reply arriving in the same second as
  the post it answers could sort under it, and a late arrival landed behind a client's since_id. A post published
  within the last hour now gets an id for its arrival; backfill keeps its published time.
- NoteParser.Time returned default(DateTime) for a missing "updated", so every remote post was stored as edited in
  year one. Migration _006 clears the stored ones.
- published now carries milliseconds, so peers that derive ids from it (GoToSocial, Mastodon) keep our posts in
  order within a second.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub.Tests/Federation/NoteParserTests.cs

@@ -54,6 +54,7 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal("a cat", media.Description);
 			Assert.Equal(new[] { 0.5f, -1f }, media.Focus);
 			Assert.Equal(new DateTime(2026, 9, 30, 10, 0, 0, DateTimeKind.Utc), note.Published);
+			Assert.Null(note.Updated);
 		}
 
 		[Fact]

PrivaPub.Tests/Federation/RendererTests.cs

@@ -57,7 +57,7 @@ namespace PrivaPub.Tests.Federation
 				Language = "en",
 				Mentions = new() { new PostMention { ActorURI = "https://m.example/users/bob", Handle = "@bob@m.example" } },
 				Tags = new() { "cats" },
-				CreationDate = new DateTime(2026, 9, 30, 17, 45, 12, DateTimeKind.Utc)
+				CreationDate = new DateTime(2026, 9, 30, 17, 45, 12, 345, DateTimeKind.Utc)
 			};
 
 			var note = ActivityPubRenderer.Note(post, Alice, default, "https://m.example/users/bob/statuses/1");
@@ -67,7 +67,7 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal("<p><strong>On &lt;cats&gt;</strong></p><p>body</p>", note["content"]!.GetValue<string>());
 			Assert.Equal("On <cats>", note["summary"]!.GetValue<string>());
 			Assert.True(note["sensitive"]!.GetValue<bool>());
-			Assert.Equal("2026-09-30T17:45:12Z", note["published"]!.GetValue<string>());
+			Assert.Equal("2026-09-30T17:45:12.345Z", note["published"]!.GetValue<string>());
 			Assert.Equal(new[] { ActivityPubRenderer.Public }, note["to"]!.AsArray().Select(n => n!.GetValue<string>()));
 			Assert.Equal(new[] { Alice.Followers, "https://m.example/users/bob" }, note["cc"]!.AsArray().Select(n => n!.GetValue<string>()));
 			var tags = note["tag"]!.AsArray();

PrivaPub.Tests/Infrastructure/PrivacyIdsTests.cs

@@ -33,6 +33,22 @@ namespace PrivaPub.Tests.Infrastructure
 			Assert.Equal(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc), ObjectId.Parse(older).CreationTime);
 		}
 
+		[Fact]
+		public void A_post_arriving_live_sorts_after_everything_already_stored()
+		{
+			var local = ObjectId.GenerateNewId().ToString();
+			var reply = PrivacyIds.Arrived(DateTime.UtcNow.AddSeconds(-30));
+			var skewed = PrivacyIds.Arrived(DateTime.UtcNow.AddYears(50));
+
+			Assert.True(string.CompareOrdinal(local, reply) < 0);
+			Assert.True(string.CompareOrdinal(reply, skewed) < 0);
+			Assert.True(ObjectId.Parse(skewed).CreationTime < DateTime.UtcNow.AddMinutes(1));
+		}
+
+		[Fact]
+		public void A_backfilled_post_keeps_its_published_time() =>
+			Assert.Equal(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc), ObjectId.Parse(PrivacyIds.Arrived(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc))).CreationTime);
+
 		[Fact]
 		public void A_published_time_in_the_future_is_clamped() =>
 			Assert.True(ObjectId.Parse(PrivacyIds.At(DateTime.UtcNow.AddYears(50))).CreationTime < DateTime.UtcNow.AddDays(2));

PrivaPub/Federation/Inbox/Handlers/AnnounceHandler.cs

@@ -71,7 +71,7 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				: DateTime.UtcNow;
 			var reblog = new PostEntity
 			{
-				ID = PrivacyIds.At(published),
+				ID = PrivacyIds.Arrived(published),
 				ObjectURI = announceId,
 				ActivityURI = announceId,
 				ActorURI = actor.ActorURI,

PrivaPub/Federation/Inbox/RemotePosts.cs

@@ -60,7 +60,7 @@ namespace PrivaPub.Federation.Inbox
 
 			return new PostEntity
 			{
-				ID = PrivacyIds.At(note.Published),
+				ID = PrivacyIds.Arrived(note.Published),
 				ObjectURI = note.Id,
 				ActorURI = author.ActorURI,
 				AuthorAccountId = author.ID,

PrivaPub/Federation/Objects/NoteParser.cs

@@ -163,7 +163,7 @@ namespace PrivaPub.Federation.Objects
 		static DateTime? Time(string value) =>
 			DateTimeOffset.TryParse(value, CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal, out var parsed)
 				? parsed.UtcDateTime
-				: default;
+				: (DateTime?)null;
 
 		static int? Int(JsonNode node) =>
 			node is JsonValue value && value.TryGetValue<int>(out var number) && number is > 0 and < 100_000 ? number : (int?)null;

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -48,7 +48,7 @@ namespace PrivaPub.Federation.Rendering
 			string.IsNullOrEmpty(markdown) ? string.Empty : Markdown.ToHtml(markdown, Pipeline).Trim();
 
 		public static string Timestamp(DateTime value) =>
-			DateTime.SpecifyKind(value, DateTimeKind.Utc).ToString("yyyy-MM-ddTHH:mm:ssZ", CultureInfo.InvariantCulture);
+			DateTime.SpecifyKind(value, DateTimeKind.Utc).ToString("yyyy-MM-ddTHH:mm:ss.fffZ", CultureInfo.InvariantCulture);
 
 		public static string Day(DateTime value) =>
 			DateTime.SpecifyKind(value, DateTimeKind.Utc).Date.ToString("yyyy-MM-ddT00:00:00Z", CultureInfo.InvariantCulture);

PrivaPub/Infrastructure/Data/Migrations/_006_unedited_remote_posts_lose_year_one.cs

@@ -0,0 +1,26 @@
+using MongoDB.Entities;
+
+using PrivaPub.Models.Post;
+
+namespace PrivaPub.Infrastructure.Data.Migrations
+{
+	public class _006_unedited_remote_posts_lose_year_one : IMigration
+	{
+		public async Task UpgradeAsync()
+		{
+			var posts = await DB.Default.Find<Post>()
+				.Match(p => p.EditedAt == DateTime.MinValue || p.UpdateDate == DateTime.MinValue || p.Revisions.Any(r => r.EditedAt == DateTime.MinValue))
+				.ExecuteAsync();
+			foreach (var post in posts)
+			{
+				foreach (var revision in post.Revisions.Where(r => r.EditedAt == DateTime.MinValue))
+					revision.EditedAt = post.CreationDate;
+				await DB.Default.Update<Post>().MatchID(post.ID)
+					.Modify(p => p.EditedAt, post.EditedAt == DateTime.MinValue ? null : post.EditedAt)
+					.Modify(p => p.UpdateDate, post.UpdateDate == DateTime.MinValue ? post.CreationDate : post.UpdateDate)
+					.Modify(p => p.Revisions, post.Revisions)
+					.ExecuteAsync();
+			}
+		}
+	}
+}

PrivaPub/Infrastructure/Ids/PrivacyIds.cs

@@ -1,12 +1,19 @@
 using System.Buffers.Binary;
 using System.Security.Cryptography;
 
+using MongoDB.Bson;
+
 namespace PrivaPub.Infrastructure.Ids
 {
 	public static class PrivacyIds
 	{
 		public static string ForDay(DateTime when) => At(DateTime.SpecifyKind(when, DateTimeKind.Utc).Date);
 
+		public static string Arrived(DateTime published) =>
+			DateTime.SpecifyKind(published, DateTimeKind.Utc) > DateTime.UtcNow - LiveWindow ? ObjectId.GenerateNewId().ToString() : At(published);
+
+		static readonly TimeSpan LiveWindow = TimeSpan.FromHours(1);
+
 		public static string At(DateTime when)
 		{
 			var seconds = Math.Clamp(new DateTimeOffset(DateTime.SpecifyKind(when, DateTimeKind.Utc)).ToUnixTimeSeconds(), 0,