Fixed #1967.

app/schemas/models/earned_achievement.coffee

@@ -27,7 +27,7 @@ module.exports =
       triggeredBy: c.objectId()
       achievementName: type: 'string'
       created: type: 'date'
-      changed: type: 'date'
+      changed: type: ['date', 'string', 'number'] # TODO: migrate timestamps and Date objects all to ISO strings 
       achievedAmount: type: 'number'
       earnedPoints: type: 'number'
       previouslyAchievedAmount: {type: 'number'}

server/achievements/EarnedAchievement.coffee

@@ -10,7 +10,7 @@ EarnedAchievementSchema = new mongoose.Schema({
 }, {strict:false})
 
 EarnedAchievementSchema.pre 'save', (next) ->
-  @set('changed', Date.now())
+  @set('changed', new Date())
   next()
 
 EarnedAchievementSchema.index({user: 1, achievement: 1}, {unique: true, name: 'earned achievement index'})

server/levels/sessions/LevelSession.coffee

@@ -23,7 +23,7 @@ LevelSessionSchema.post 'init', (doc) ->
 
 LevelSessionSchema.pre 'save', (next) ->
   User = require '../../users/User'  # Avoid mutual inclusion cycles
-  @set('changed', new Date())
+  @set('changed', new Date().toISOString())
 
   id = @get('id')
   initd = id of previous