Allow conversion from date strings to "unix" timestamp. (#1985)

"unix" timestamp is number of seconds since Jan 1, 1970 UTC.
This allows scripts that use templates to generate time
deltas in seconds if desired from state attributes such
as last_updated.

Some examples:

timestamp now is
{{ as_timestamp(now) }}

timstamp of last change is
{{ as_timestamp(states.binary_sensor.garage_door.last_changed) }}

seconds since last change is
{{ as_timestamp(now) - as_timestamp(states.binary_sensor.garage_door.last_changed) }}

homeassistant/util/dt.py

@@ -1,4 +1,5 @@
 """Provides helper methods to handle the time in HA."""
+import calendar
 import datetime as dt
 import re
 
@@ -59,6 +60,17 @@ def as_utc(dattim):
     return dattim.astimezone(UTC)
 
 
+def as_timestamp(dt_value):
+    """Convert a date/time into a unix time (seconds since 1970)."""
+    if hasattr(dt_value, "utctimetuple"):
+        parsed_dt = dt_value
+    else:
+        parsed_dt = parse_datetime(str(dt_value))
+        if not parsed_dt:
+            raise ValueError("not a valid date/time.")
+    return calendar.timegm(parsed_dt.utctimetuple())
+
+
 def as_local(dattim):
     """Convert a UTC datetime object to local time zone."""
     if dattim.tzinfo == DEFAULT_TIME_ZONE: