Convert creation/modification dates to UTC (fixes #155)

Ensure that timezones are correctly interpreted in the input by calling
`.astimezone()` as appropriate on datetime objects, and store the
resulting date fields as UTC.

One could argue that datetimes in the local timezone be stored in the
PDF, but then the date string handling becomes more complicated; the PDF
and XMP date specs both use the `Z` suffix to indicate UTC time, but
other +/- offsets require different syntax between the two specs.

src/img2pdf.py

@@ -36,7 +36,7 @@ if hasattr(GifImagePlugin, "LoadingStrategy"):
 
 # TiffImagePlugin.DEBUG = True
 from PIL.ExifTags import TAGS
-from datetime import datetime
+from datetime import datetime, timezone
 from jp2 import parsejp2
 from enum import Enum
 from io import BytesIO
@@ -722,7 +722,7 @@ class pdfdoc(object):
             self.writer.docinfo = PdfDict(indirect=True)
 
         def datetime_to_pdfdate(dt):
-            return dt.strftime("%Y%m%d%H%M%SZ")
+            return dt.astimezone(tz=timezone.utc).strftime("%Y%m%d%H%M%SZ")
 
         for k in ["Title", "Author", "Creator", "Producer", "Subject"]:
             v = locals()[k.lower()]
@@ -732,7 +732,7 @@ class pdfdoc(object):
                 v = PdfString.encode(v)
             self.writer.docinfo[getattr(PdfName, k)] = v
 
-        now = datetime.now()
+        now = datetime.now().astimezone()
         for k in ["CreationDate", "ModDate"]:
             v = locals()[k.lower()]
             if v is None and nodate:
@@ -752,7 +752,7 @@ class pdfdoc(object):
                 )
 
         def datetime_to_xmpdate(dt):
-            return dt.strftime("%Y-%m-%dT%H:%M:%SZ")
+            return dt.astimezone(tz=timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")
 
         self.xmp = b"""<?xpacket begin='\xef\xbb\xbf' id='W5M0MpCehiHzreSzNTczkc9d'?>
 <x:xmpmeta xmlns:x='adobe:ns:meta/' x:xmptk='XMP toolkit 2.9.1-13, framework 1.6'>