Improve logging if future process discovered

Before this change we only logged that it happened.

With this change in place, we log how we decide that it is a future
process.

px/px_process.py

@@ -1,3 +1,4 @@
+import logging
 import datetime
 import operator
 import subprocess
@@ -21,6 +22,9 @@
     from six import text_type      # NOQA
 
 
+LOG = logging.getLogger(__name__)
+
+
 # Match + group: " 7708 1 Mon Mar  7 09:33:11 2016  netbios 0.1 0:00.08  0.0 /usr/sbin/netbiosd hj"
 PS_LINE = re.compile(
     " *([0-9]+) +([0-9]+) +([A-Za-z0-9: ]+) +([^ ]+) +([0-9.]+) +([-0-9.:]+) +([0-9.]+) +(.*)")
@@ -65,6 +69,14 @@ def __init__(self,
         time = datetime.datetime.strptime(start_time_string.strip(), "%c")
         self.start_time = time.replace(tzinfo=TIMEZONE)  # type: datetime.datetime
         self.age_seconds = (now - self.start_time).total_seconds()  # type: float
+        if self.age_seconds < 0:
+            LOG.error("Process age < 0: age_seconds=%r now=%r start_time=%r start_time_string=%r timezone=%r",
+                self.age_seconds,
+                now,
+                self.start_time,
+                start_time_string.strip(),
+                datetime.datetime.now(TIMEZONE).tzname())
+            assert False
         assert self.age_seconds >= 0
         self.age_s = seconds_to_str(self.age_seconds)  # type: text_type