Fix start time, add early start latency to start a bit earlier

SAS/TMSS/backend/src/tmss/tmssapp/adapters/l2stationspecs.py

@@ -44,12 +44,19 @@ schema = """
     },
     "start_time": {
       "type": "string",
-      "format": "date-time"
+      "format": "date-time",
+      "default": "1970-01-01T00:00:00"
     },
     "stop_time": {
       "type": "string",
       "format": "date-time"
     },
+    "early_start_latency": {
+      "type": "number",
+      "description": "Number of seconds to start before the provided start time, to account for initialising the on-line signal chain, and for possibly negative geometrical delay compensation.",
+      "default": 2.0,
+      "minimum": 0
+    },
     "antenna_field": {
       "default": "HBA",
       "description": "Antenna field to use",