diff --git a/ogn/exceptions.py b/ogn/exceptions.py
index 3a98ff5..fcbb6c1 100644
--- a/ogn/exceptions.py
+++ b/ogn/exceptions.py
@@ -1,20 +1,34 @@
 """
 exception definitions
 """
+from datetime import datetime
 
 
 class AprsParseError(Exception):
     """Parse error while parsing an aprs packet."""
     def __init__(self, aprs_string):
-        self.message = "This is not a valid APRS string: %s" % aprs_string
-        super(AprsParseError, self).__init__(self.message)
         self.aprs_string = aprs_string
 
+        self.message = "This is not a valid APRS string: {}".format(aprs_string)
+        super(AprsParseError, self).__init__(self.message)
+
 
 class OgnParseError(Exception):
-    """Parse error while parsing an aprs packet substring"""
+    """Parse error while parsing an aprs packet substring."""
     def __init__(self, substring, expected_type):
-        self.message = "For type %s this is not a valid token: %s" % (expected_type, substring)
-        super(OgnParseError, self).__init__(self.message)
         self.substring = substring
         self.expected_type = expected_type
+
+        self.message = "For type {} this is not a valid token: {}".format(expected_type, substring)
+        super(OgnParseError, self).__init__(self.message)
+
+
+class AmbigousTimeError(Exception):
+    """Timstamp from the past/future, can't fully reconstruct datetime from timestamp."""
+    def __init__(self, reference, packet_time):
+        self.reference = reference
+        self.packet_time = packet_time
+        self.timedelta = reference - datetime.combine(reference, packet_time)
+
+        self.message = "Can't reconstruct timstamp, {:.0f}s from past.".format(self.timedelta.total_seconds())
+        super(AmbigousTimeError, self).__init__(self.message)