Reset storageModTime to 0 if < 0 and prevent setting < 0, though it shouldn't happen to begin with

chrome/content/zotero/xpcom/schema.js

@@ -2443,6 +2443,10 @@ Zotero.Schema = new function(){
 					Zotero.DB.query("UPDATE itemNotes SET sourceItemID=NULL WHERE sourceItemID=itemID");
 				}
 				
+				if (i==61) {
+					Zotero.DB.query("UPDATE itemAttachments SET storageModTime=NULL WHERE storageModTime<0");
+				}
+				
 				Zotero.wait();
 			}
 			

chrome/content/zotero/xpcom/storage.js

@@ -343,6 +343,12 @@ Zotero.Sync.Storage = new function () {
 	 *												attachment item
 	 */
 	this.setSyncedModificationTime = function (itemID, mtime, updateItem) {
+		if (mtime < 0) {
+			Components.utils.reportError("Invalid file mod time " + mtime
+				+ " in Zotero.Storage.setSyncedModificationTime()");
+			mtime = 0;
+		}
+		
 		Zotero.DB.beginTransaction();
 		
 		var sql = "UPDATE itemAttachments SET storageModTime=? WHERE itemID=?";

userdata.sql

@@ -1,4 +1,4 @@
--- 60
+-- 61
 
 -- This file creates tables containing user-specific data for new users --
 -- any changes made here must be mirrored in transition steps in schema.js::_migrateSchema()