Ignore unix timestamp markers with invalid offsets

Fixes #681

DiscordChatExporter.Core/Discord/Data/User.cs

@@ -71,4 +71,4 @@ namespace DiscordChatExporter.Core.Discord.Data
             return new User(id, isBot, discriminator, name, avatarUrl);
         }
     }
-}
+}

DiscordChatExporter.Core/Markdown/Parsing/MarkdownParser.cs

@@ -238,8 +238,8 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
             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.
+                // TODO: support formatting parameters
+                // See: https://github.com/Tyrrrz/DiscordChatExporter/issues/662
 
                 if (!long.TryParse(m.Groups[1].Value, NumberStyles.Integer, CultureInfo.InvariantCulture,
                     out var offset))
@@ -247,6 +247,13 @@ namespace DiscordChatExporter.Core.Markdown.Parsing
                     return null;
                 }
 
+                // Bound check
+                // https://github.com/Tyrrrz/DiscordChatExporter/issues/681
+                if (offset < TimeSpan.MinValue.TotalSeconds || offset > TimeSpan.MaxValue.TotalSeconds)
+                {
+                    return null;
+                }
+
                 return new UnixTimestampNode(DateTimeOffset.UnixEpoch + TimeSpan.FromSeconds(offset));
             }
         );