--- a/agl.py	Mon Sep 11 22:09:37 2017 +0930
+++ b/agl.py	Mon Sep 11 22:14:37 2017 +0930
@@ -44,6 +44,9 @@
     if not args.update and not args.graph:
         parser.error('Nothing to do')
 
+    if args.days is not None and args.days < 0:
+        parser.error('days must be non-negative')
+
     start = args.start
     if start is None:
         start = datetime.date.today()
@@ -57,6 +60,9 @@
         end = start + datetime.timedelta(days = 1)
         end = datetime.datetime(end.year, end.month, end.day)
 
+    if start >= end:
+        parser.error('Start must be before end')
+
     dbh = sqlite3.connect(dbfn, detect_types = sqlite3.PARSE_DECLTYPES)
     cur = dbh.cursor()
     if args.update: