Handle files which only have local/relative timestamps

In some files, perhaps created for non-GPS activities or by non-GPS devices,
all timestamps in laps and trackpoints are expressed as integer offsets.
(Probably an offset in seconds relative to when the device was turned on.)

The only "real" timestamp available is in the 'activity' message field
called 'local_timestamp', which represents the device's time in its *local*
timezone.

To process such files, we must use 'activity.local_timestamp' along with
'activity.timestamp' to calibrate the offset between the integer timestamps
and Unix epoch seconds.  However, even after doing this we *do not know* the
correct timezone for the resulting timestamps.  We emphasize this in the
ISO8601-format timestamps in the TCX output, by not appending 'Z'.

'tests/files/antfs-dump.63.fit' from dtcooper/python-fitparse is a good
example of this issue.  It contains data from a heart-rate monitor device
without GPS.

fittotcx/program.py

@@ -27,6 +27,7 @@
 import argparse
 import sys
 import lxml.etree
+from datetime import datetime
 from fitparse import FitFile, FitParseError
 
 try:
@@ -82,6 +83,15 @@ def ff(number):
     return "{:.10f}".format(number).rstrip("0").rstrip(".")
 
 
+def tsz(number_or_datetime, epoch_offset):
+    if number_or_datetime is None:
+        return None, ""
+    elif isinstance(number_or_datetime, int):
+        return datetime.fromtimestamp(number_or_datetime + epoch_offset), ""
+    else:
+        return number_or_datetime, "Z"
+
+
 def create_element(tag, text=None, namespace=None):
     namespace = NSMAP[namespace]
     tag = "{%s}%s" % (namespace, tag)
@@ -158,8 +168,8 @@ def add_author(document):
     create_sub_element(b, "BuildMinor", "0")
 
 
-def add_trackpoint(element, trackpoint):
-    timestamp = trackpoint.get_value("timestamp")
+def add_trackpoint(element, trackpoint, epoch_offset=None):
+    timestamp, zone = tsz(trackpoint.get_value("timestamp"), epoch_offset)
     pos_lat = trackpoint.get_value("position_lat")
     pos_long = trackpoint.get_value("position_long")
     distance = trackpoint.get_value("distance")
@@ -168,7 +178,7 @@ def add_trackpoint(element, trackpoint):
     heart_rate = trackpoint.get_value("heart_rate")
     cadence = trackpoint.get_value("cadence")
 
-    create_sub_element(element, "Time", timestamp.isoformat() + "Z")
+    create_sub_element(element, "Time", timestamp.isoformat() + zone)
 
     if pos_lat is not None and pos_long is not None:
         pos = create_sub_element(element, "Position")
@@ -200,9 +210,9 @@ def add_trackpoint(element, trackpoint):
         create_sub_element(tpx, "Speed", ff(speed))
 
 
-def add_lap(element, activity, lap):
-    start_time = lap.get_value("start_time")
-    end_time = lap.get_value("timestamp")  # opt
+def add_lap(element, activity, lap, epoch_offset=None):
+    start_time, zone = tsz(lap.get_value("start_time"), epoch_offset)
+    end_time, _ = tsz(lap.get_value("timestamp"), epoch_offset)  # opt
 
     totaltime = lap.get_value("total_elapsed_time")  # opt
     if totaltime is None:
@@ -227,7 +237,7 @@ def add_lap(element, activity, lap):
     # extensions
 
     lapelem = create_sub_element(element, "Lap")
-    lapelem.set("StartTime", start_time.isoformat() + "Z")
+    lapelem.set("StartTime", start_time.isoformat() + zone)
 
     if totaltime is not None:
         create_sub_element(lapelem, "TotalTimeSeconds", ff(totaltime))
@@ -249,11 +259,11 @@ def add_lap(element, activity, lap):
     # Add track points to lap
     trackelem = create_sub_element(lapelem, "Track")
     for trackpoint in activity.get_messages(name="record"):
-        tts = trackpoint.get_value("timestamp")
+        tts = tsz(trackpoint.get_value("timestamp"), epoch_offset)
         if start_time <= tts:
             if end_time is None or tts <= end_time:
                 trackpointelem = create_sub_element(trackelem, "Trackpoint")
-                add_trackpoint(trackpointelem, trackpoint)
+                add_trackpoint(trackpointelem, trackpoint, epoch_offset)
 
 
 def add_activity(element, activity):
@@ -262,17 +272,30 @@ def add_activity(element, activity):
     # Sport type
     sport = SPORT_MAP.get(session.get_value("sport"), "Other")
 
-    # Identity (in UTC)
+    # Identity (along with all other FIT timestamps, normally in UTC)
+    epoch_offset, zone = None, "Z"
     identity = session.get_value("start_time")
+    if isinstance(identity, int):
+        # Try to use activity.local_timestamp as a backup (e.g. for non-GPS activities)
+        act_message = next(activity.get_messages(name="activity"), None)
+        if act_message is None:
+            raise FitParseError("FIT file contains insufficient time information")
+
+        lt = act_message.get_value("local_timestamp")
+        ts = act_message.get_value("timestamp")
+        if not isinstance(lt, datetime) or not isinstance(ts, int):
+            raise FitParseError("FIT file contains insufficient time information")
+        epoch_offset, zone = int(lt.timestamp()) - ts, ""
+        identity = lt
 
     actelem = create_sub_element(element, "Activity")
     actelem.set("Sport", sport)
-    create_sub_element(actelem, "Id", identity.isoformat() + "Z")
+    create_sub_element(actelem, "Id", identity.isoformat() + zone)
 
     for lap in activity.get_messages("lap"):
-        add_lap(actelem, activity, lap)
+        add_lap(actelem, activity, lap, epoch_offset)
 
-    device_info = next(activity.get_messages(name="device_info"))
+    device_info = next(activity.get_messages(name="device_info"), None)
     if device_info is not None:
         add_creator(actelem, device_info)