diff --git a/packages/lib/PlayerStats.js b/packages/lib/PlayerStats.js
new file mode 100644
index 00000000..19e36818
--- /dev/null
+++ b/packages/lib/PlayerStats.js
@@ -0,0 +1,84 @@
+"use strict";
+
+/**
+ * Stastics collected about a player.
+ * @alias module:lib/PlayerStats
+ */
+class PlayerStats {
+	/**
+	 * Count of the number of times this player has been seen joining this server.
+	 * @type {number}
+	 */
+	joinCount = 0;
+
+	/**
+	 * Time in ms this player has been seen online on this instance.
+	 * @type {number}
+	 */
+	onlineTimeMs = 0;
+
+	/**
+	 * Timestamp the player was last seen joining.
+	 * @type {Date=}
+	 */
+	lastJoinAt;
+
+	/**
+	 * Timestamp the player was last seen leaving.
+	 * @type {Date=}
+	 */
+	lastLeaveAt;
+
+	/**
+	 * Reason the player was last seen leaving with.
+	 * @type {string=}
+	 */
+	lastLeaveReason;
+
+	static jsonSchema = {
+		type: "object",
+		properties: {
+			"join_count": { type: "integer" },
+			"online_time_ms": { type: "number" },
+			"last_join_at": { type: "number" },
+			"last_leave_at": { type: "number" },
+			"last_leave_reason": { type: "string" },
+		},
+	};
+
+	constructor(json = {}) {
+		if (json["join_count"]) {
+			this.joinCount = json["join_count"];
+		}
+		if (json["online_time_ms"]) {
+			this.onlineTimeMs = json["online_time_ms"];
+		}
+		if (json["last_join_at"]) {
+			this.lastJoinAt = new Date(json["last_join_at"]);
+		}
+		if (json["last_leave_at"]) {
+			this.lastLeaveAt = new Date(json["last_leave_at"]);
+			this.lastLeaveReason = json["last_leave_reason"] || "quit";
+		}
+	}
+
+	toJSON() {
+		let json = {};
+		if (this.joinCount) {
+			json["join_count"] = this.joinCount;
+		}
+		if (this.onlineTimeMs) {
+			json["online_time_ms"] = this.onlineTimeMs;
+		}
+		if (this.lastJoinAt) {
+			json["last_join_at"] = this.lastJoinAt.getTime();
+		}
+		if (this.lastLeaveAt) {
+			json["last_leave_at"] = this.lastLeaveAt.getTime();
+			json["last_leave_reason"] = this.lastLeaveReason;
+		}
+		return json;
+	}
+}
+
+module.exports = PlayerStats;
diff --git a/packages/slave/src/Instance.js b/packages/slave/src/Instance.js
index 9207cad9..1fa1543c 100644
--- a/packages/slave/src/Instance.js
+++ b/packages/slave/src/Instance.js
@@ -9,6 +9,7 @@ const util = require("util");
 const libFileOps = require("@clusterio/lib/file_ops");
 const libFactorio = require("@clusterio/lib/factorio");
 const libLink = require("@clusterio/lib/link");
+const PlayerStats = require("@clusterio/lib/PlayerStats");
 const libPlugin = require("@clusterio/lib/plugin");
 const libPluginLoader = require("@clusterio/lib/plugin_loader");
 const libErrors = require("@clusterio/lib/errors");
@@ -254,29 +255,9 @@ class Instance extends libLink.Link {
 			}
 		});
 
-		/**
-		 * @typedef module:slave/src/Instance~PlayerStat
-		 * @type {object}
-		 * @property {number} lastJoinAt -
-		 *     Unix timestamp in ms the player was last seen joining this
-		 *     instance.
-		 * @property {number=} lastLeaveAt -
-		 *     Unix timestamp in ms the player was last seen leaving this
-		 *     instance.  Not present if player has yet to leave after first
-		 *     time joining.
-		 * @property {string=} lastLeaveReason -
-		 *     Reason the player was last seen leaving with.  Not present if
-		 *     player has yet to leave after first time joining.
-		 * @property {number} joinCount -
-		 *     Count of the number of times this player has been seen
-		 *     joining this server.
-		 * @property {number} onlineTimeMs -
-		 *     Time in ms this player has been seen online on this instance.
-		 */
-
 		/**
 		 * Per player statistics recorded by this instance.
-		 * @type {Map<string, module:slave/src/Instance~PlayerStat>}
+		 * @type {Map<string, module:lib/PlayerStats>}
 		 */
 		this.playerStats = new Map();
 
@@ -351,10 +332,10 @@ class Instance extends libLink.Link {
 
 		let stats = this.playerStats.get(name);
 		if (!stats) {
-			stats = { joinCount: 0, onlineTimeMs: 0 };
+			stats = new PlayerStats();
 			this.playerStats.set(name, stats);
 		}
-		stats.lastJoinAt = Date.now();
+		stats.lastJoinAt = new Date();
 		stats.joinCount += 1;
 
 		let event = {
@@ -372,9 +353,9 @@ class Instance extends libLink.Link {
 		}
 
 		let stats = this.playerStats.get(name);
-		stats.lastLeaveAt = Date.now();
+		stats.lastLeaveAt = new Date();
 		stats.lastLeaveReason = reason;
-		stats.onlineTimeMs += stats.lastLeaveAt - stats.lastJoinAt;
+		stats.onlineTimeMs += stats.lastLeaveAt.getTime() - stats.lastJoinAt.getTime();
 		this._hadPlayersOnline = true;
 
 		let event = {
@@ -517,17 +498,17 @@ class Instance extends libLink.Link {
 	}
 
 	async _loadStats() {
-		let stats;
+		let instanceStats;
 		try {
-			stats = JSON.parse(await fs.readFile(this.path("instance-stats.json")));
+			instanceStats = JSON.parse(await fs.readFile(this.path("instance-stats.json")));
 		} catch (err) {
 			if (err.code === "ENOENT") {
 				return;
 			}
 			throw err;
 		}
-		this.playerStats = new Map(stats["players"]);
-		this._playerAutosaveSlot = stats["player_autosave_slot"] || 1;
+		this.playerStats = new Map(instanceStats["players"].map(([id, stats]) => [id, new PlayerStats(stats)]));
+		this._playerAutosaveSlot = instanceStats["player_autosave_slot"] || 1;
 	}
 
 	async _saveStats() {