Add support for timestamp markers

Closes #637

DiscordChatExporter.Core/Markdown/Parsing/MarkdownParser.cs

@@ -1,4 +1,6 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using System.Text.RegularExpressions;
 using DiscordChatExporter.Core.Utils;
@@ -225,6 +227,26 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
             (_, m) => new TextNode(m.Groups[1].Value)
         );
 
+        /* Misc */
+
+        // Capture <t:12345678> or <t:12345678:R>
+        private static readonly IMatcher<MarkdownNode> UnixTimestampNodeMatcher = new RegexMatcher<MarkdownNode>(
+            new Regex("<t:(\\d+)(?::\\w)?>", DefaultRegexOptions),
+            (_, m) =>
+            {
+                // We don't care about the 'R' parameter because we're not going to
+                // show relative timestamps in an export anyway.
+
+                if (!long.TryParse(m.Groups[1].Value, NumberStyles.Integer, CultureInfo.InvariantCulture,
+                    out var offset))
+                {
+                    return null;
+                }
+
+                return new UnixTimestampNode(DateTimeOffset.UnixEpoch + TimeSpan.FromSeconds(offset));
+            }
+        );
+
         // Combine all matchers into one
         // Matchers that have similar patterns are ordered from most specific to least specific
         private static readonly IMatcher<MarkdownNode> AggregateNodeMatcher = new AggregateMatcher<MarkdownNode>(
@@ -266,7 +288,10 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
             // Emoji
             StandardEmojiNodeMatcher,
             CustomEmojiNodeMatcher,
-            CodedStandardEmojiNodeMatcher
+            CodedStandardEmojiNodeMatcher,
+
+            // Misc
+            UnixTimestampNodeMatcher
         );
 
         // Minimal set of matchers for non-multimedia formats (e.g. plain text)
@@ -279,7 +304,10 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
             RoleMentionNodeMatcher,
 
             // Emoji
-            CustomEmojiNodeMatcher
+            CustomEmojiNodeMatcher,
+
+            // Misc
+            UnixTimestampNodeMatcher
         );
 
         private static IReadOnlyList<MarkdownNode> Parse(StringPart stringPart, IMatcher<MarkdownNode> matcher) =>

DiscordChatExporter.Core/Markdown/Parsing/MarkdownVisitor.cs

@@ -5,7 +5,8 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
 {
     internal abstract class MarkdownVisitor
     {
-        protected virtual MarkdownNode VisitText(TextNode text) => text;
+        protected virtual MarkdownNode VisitText(TextNode text) =>
+            text;
 
         protected virtual MarkdownNode VisitFormatted(FormattedNode formatted)
         {
@@ -13,15 +14,23 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
             return formatted;
         }
 
-        protected virtual MarkdownNode VisitInlineCodeBlock(InlineCodeBlockNode inlineCodeBlock) => inlineCodeBlock;
+        protected virtual MarkdownNode VisitInlineCodeBlock(InlineCodeBlockNode inlineCodeBlock) =>
+            inlineCodeBlock;
 
-        protected virtual MarkdownNode VisitMultiLineCodeBlock(MultiLineCodeBlockNode multiLineCodeBlock) => multiLineCodeBlock;
+        protected virtual MarkdownNode VisitMultiLineCodeBlock(MultiLineCodeBlockNode multiLineCodeBlock) =>
+            multiLineCodeBlock;
 
-        protected virtual MarkdownNode VisitLink(LinkNode link) => link;
+        protected virtual MarkdownNode VisitLink(LinkNode link) =>
+            link;
 
-        protected virtual MarkdownNode VisitEmoji(EmojiNode emoji) => emoji;
+        protected virtual MarkdownNode VisitEmoji(EmojiNode emoji) =>
+            emoji;
 
-        protected virtual MarkdownNode VisitMention(MentionNode mention) => mention;
+        protected virtual MarkdownNode VisitMention(MentionNode mention) =>
+            mention;
+
+        protected virtual MarkdownNode VisitUnixTimestamp(UnixTimestampNode timestamp) =>
+            timestamp;
 
         public MarkdownNode Visit(MarkdownNode node) => node switch
         {
@@ -32,6 +41,7 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
             LinkNode link => VisitLink(link),
             EmojiNode emoji => VisitEmoji(emoji),
             MentionNode mention => VisitMention(mention),
+            UnixTimestampNode timestamp => VisitUnixTimestamp(timestamp),
             _ => throw new ArgumentOutOfRangeException(nameof(node))
         };
 

DiscordChatExporter.Core/Markdown/UnixTimestampNode.cs

@@ -0,0 +1,13 @@
+using System;
+
+namespace DiscordChatExporter.Core.Markdown
+{
+    internal class UnixTimestampNode : MarkdownNode
+    {
+        public DateTimeOffset Value { get; }
+
+        public UnixTimestampNode(DateTimeOffset value) => Value = value;
+
+        public override string ToString() => Value.ToString();
+    }
+}