Fix #409 ts.utc() would die if given a date object

The routine would die because `combine()` is a class method, not a
module method; and because it needed a timezone.  A test is here added
to keep the feature fixed.  Also, the timelib module used `dt` for the
`datetime` module but also sometimes as a convenient local name for a
`datetime`, so let’s rename the module to `dt_module` for readability
and consistency.  Inspired by the attempted fix in #410.

skyfield/tests/test_timelib.py

@@ -1,3 +1,4 @@
+import datetime as dt_module
 import numpy as np
 from assay import assert_raises
 from pytz import timezone
@@ -102,6 +103,11 @@ def test_building_time_from_list_of_utc_datetimes(ts):
         2442046.5, 2442047.5, 2442048.5, 2442049.5, 2442050.5, 2442051.5,
     ]
 
+def test_building_time_from_python_date(ts):
+    d = dt_module.date(2020, 7, 22)
+    t = ts.utc(d)
+    assert t.utc == (2020, 7, 22, 0, 0, 0.0)
+
 def test_converting_ut1_to_tt(ts):
     ten_thousand_years = 365 * 10000
 

skyfield/timelib.py

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-import datetime as dt
+import datetime as dt_module
 import re
 from collections import namedtuple
 from datetime import date, datetime
@@ -33,8 +33,8 @@ class CalendarArray(ndarray):
     @property
     def second(self): return self[5]
 
-if hasattr(dt, 'timezone'):
-    utc = dt.timezone.utc
+if hasattr(dt_module, 'timezone'):
+    utc = dt_module.timezone.utc
 else:
     try:
         from pytz import utc
@@ -42,9 +42,9 @@ else:
         # Lacking a full suite of timezones from pytz, we at least need a
         # time zone object for UTC.
 
-        class UTC(dt.tzinfo):
+        class UTC(dt_module.tzinfo):
             'UTC'
-            zero = dt.timedelta(0)
+            zero = dt_module.timedelta(0)
             def utcoffset(self, dt):
                 return self.zero
             def tzname(self, dt):
@@ -56,7 +56,7 @@ else:
 
 # Much of the following code is adapted from the USNO's "novas.c".
 
-_time_zero = dt.time()
+_time_zero = dt_module.time(tzinfo=utc)
 _half_minute = 30.0 / DAY_S
 _half_second = 0.5 / DAY_S
 _half_microsecond = 0.5e-6 / DAY_S
@@ -150,7 +150,7 @@ class Timescale(object):
         if isinstance(year, datetime):
             return self.from_datetime(year)
         if isinstance(year, date):
-            return self.from_datetime(dt.combine(year, _time_zero))
+            return self.from_datetime(datetime.combine(year, _time_zero))
         if hasattr(year, '__len__') and isinstance(year[0], datetime):
             return self.from_datetimes(year)