wacom: set the device time to UTC time on pairing

And let us handle the local time zone conversions in the client, because
having timestamps in localtime in the daemon is confusing and error-prone (how
is the client supposed to know what localtime was?)

README.md

@@ -139,7 +139,9 @@ org.freedesktop.tuhi1.Device
 
   Property: DrawingsAvailable (at)
       An array of timestamps of the available drawings. The timestamp of
-      each drawing can be used as argument to GetJSONData().
+      each drawing can be used as argument to GetJSONData(). Timestamps are
+      in seconds since the Epoch and may be used to display information to
+      the user or sort data.
       Read-only
 
   Property: Listening (b)

tools/tuhi-kete.py

@@ -502,7 +502,7 @@ class Fetcher(Worker):
         for idx in self.indices:
             jsondata = self.device.json(idx)
             data = json.loads(jsondata)
-            t = time.gmtime(data['timestamp'])
+            t = time.localtime(data['timestamp'])
             t = time.strftime('%Y-%m-%d-%H-%M', t)
             path = f'{data["devicename"]}-{t}.svg'
             self.json_to_svg(data, path)

tuhi/wacom.py

@@ -13,6 +13,7 @@
 
 
 import binascii
+import calendar
 import logging
 import threading
 import time
@@ -268,7 +269,8 @@ class WacomDevice(GObject.Object):
                                       expected_opcode=0xb3)
 
     def set_time(self):
-        self.current_time = time.strftime("%y%m%d%H%M%S")
+        # Device time is UTC
+        self.current_time = time.strftime("%y%m%d%H%M%S", time.gmtime())
         args = [int(i) for i in binascii.unhexlify(self.current_time)]
         self.send_nordic_command_sync(command=0xb6,
                                       expected_opcode=0xb3,
@@ -463,7 +465,7 @@ class WacomDevice(GObject.Object):
         x, y, p = 0, 0, 0
         dx, dy, dp = 0, 0, 0
 
-        timestamp = int(time.mktime(timestamp))
+        timestamp = int(calendar.timegm(timestamp))
         drawings = []
         drawing = None
         stroke = None