diff --git a/bot.ps1 b/bot.ps1
index 46726b1..ff055c8 100755
--- a/bot.ps1
+++ b/bot.ps1
@@ -26,6 +26,11 @@ if($env:UNENCRYPTED -eq 'TRUE') {
     $Unencrypted = $true
 }
 
+#$join_time contains rooms we joined during runtime
+#it is used to ignore events sent before we joined
+$join_time = @{}
+
+
 #used in Invoke-RestMethod
 $authentication_headers = @{
     Authentication = 'Bearer'
@@ -119,20 +124,28 @@ function Send-Pong {
 
     Send-MatrixEvent -RoomId $RoomId -Event $event_json -EventType 'm.room.message'
 }
-function Compare-Timestamps {
+function ConvertFrom-MatrixTimestamp {
     param (
-        [Parameter(Mandatory=$true)]
+        [Parameter(Mandatory)]
         [Int64]
         $OriginServerTs
     )
 
-    $current_time = (Get-Date).ToUniversalTime()
-
     #$OriginServerTs is milliseconds since 1970-01-01 (epoch time in milliseconds)
-    $original_time = Get-Date -Date ((Get-Date -Date '1970-01-01') + [timespan]::FromMilliseconds($OriginServerTs))
+    return Get-Date -Date ((Get-Date -Date '1970-01-01') + [timespan]::FromMilliseconds($OriginServerTs))
+}
+function Measure-TimeDifference {
+    param (
+        [Parameter(Mandatory=$true)]
+        [datetime]
+        $OriginTime,
+
+        [datetime]
+        $ReferenceTime = (Get-Date).ToUniversalTime()
+    )
 
     #return the difference
-    Write-Output ($current_time - $original_time)
+    Write-Output ($ReferenceTime - $OriginTime)
 }
 function ConvertTo-HumanReadableTimespan {
     param (
@@ -206,9 +219,23 @@ function Open-JoinEvent {
         $RoomId
     )
 
+    $origin_time = ConvertFrom-MatrixTimestamp -OriginServerTs $Event.origin_server_ts
+
+    #check if a $join_time for $RoomId exists and if it newer than our event
+    if($join_time.$RoomId) {
+        if((Measure-TimeDifference -OriginTime $origin_time -ReferenceTime $join_time.$RoomId) -gt 0) {
+            #ignore events sent before we joined the room
+            return
+        } else {
+            #first event with negative difference received
+            #probably all future events are after our join time
+            $join_time.Remove($RoomId)
+        }
+    }
+
     #the "ball" is a string returned by the bot
     if($Event.content.msgtype -eq 'm.text' -and $Event.content.body -match '^!ping( (?<ball>.*))?') {
-        $difference = Compare-Timestamps -OriginServerTs $Event.origin_server_ts
+        $difference = Measure-TimeDifference -OriginTime $origin_time
         $readable_timespan = ConvertTo-HumanReadableTimespan -TimeSpan $difference
 
         #$bodies contains a hashtable with keys Body and FormattedBody
@@ -328,6 +355,7 @@ function Invoke-MatrixSync {
     $room_ids = $response.rooms.invite.PSObject.Properties.Name
     foreach($room_id in $room_ids) {
         Join-MatrixRoom -RoomId $room_id
+        $join_time.$room_id = (Get-Date).ToUniversalTime()
     }
 
     return $response.next_batch