Fix VTEC timestamp parsing

lib/nexrad/vtec.py

@@ -26,9 +26,32 @@ RE_HYDRO = re.compile(r'''
 ''', re.X)
 
 def parse_timestamp(text: str):
-    return datetime.datetime.strptime(
-        text, '%y%m%dT%H%M%SZ'
-    ).astimezone(datetime.UTC)
+    year   = int(text[0:2])
+    month  = int(text[2:4])
+    day    = int(text[4:6])
+    hour   = int(text[7:9])
+    minute = int(text[9:11])
+
+    if year >= 70:
+        year += 1900
+    else:
+        year += 2000
+
+    if month == 0:
+        month = 1
+
+    if day == 0:
+        day = 1
+
+    return datetime.datetime(
+        year   = year,
+        month  = month,
+        day    = day,
+        hour   = hour,
+        minute = minute,
+        second = 0,
+        tzinfo = datetime.UTC
+    )
 
 class VTECEventType(enum.StrEnum):
     OPERATIONAL       = 'O'