Resolve L2SS-703 "Fix statisticswriter windows"

Closes L2SS-703

tangostationcontrol/tangostationcontrol/devices/sdp/statistics_collector.py

@@ -235,9 +235,8 @@ class XSTCollector(StatisticsCollector):
         # log if we're replacing a subband we were once recording
         previous_subband_in_slot   = self.parameters["xst_subbands"][subband_slot]
         if previous_subband_in_slot != fields.subband_index:
-            previous_subband_timestamp = datetime.datetime.fromtimestamp(self.parameters["xst_timestamps"][subband_slot])
-
-            if previous_subband_timestamp.timestamp() > 0:
+            if self.parameters["xst_timestamps"][subband_slot] > 0:
+                previous_subband_timestamp = datetime.datetime.fromtimestamp(self.parameters["xst_timestamps"][subband_slot])
                 logger.info(f"Stopped recording XSTs for subband {previous_subband_in_slot}. Last data for this subband was received at {previous_subband_timestamp}.")
    
     def xst_values(self, subband_indices = None):