journalctl should be utc; handle case where no update has taken place yet

diff_packages.py

@@ -8,7 +8,7 @@ from pathlib import Path
 
 def get_last_boot():
     with subprocess.Popen(
-        ["journalctl", "--list-boots"], stdout=subprocess.PIPE
+        ["journalctl", "--utc", "--list-boots"], stdout=subprocess.PIPE
     ) as proc:
         for line in proc.stdout.read().decode().split("\n"):
             info, end_time = line.split(chr(8212))
@@ -40,6 +40,9 @@ def get_snapshot__after_datetime(start: datetime.datetime):
             if 0 < d < td:
                 td = d
                 ret = snapshot
+        if ret is None:
+            print(f"No update since last boot on {start.astimezone()}")
+            exit(0)
         return ret