yanger: Generalize converting time deltas to YANG dates

Make sure that we have one way of getting from Python's native
datetime to a YANG compatible format.

src/statd/python/yanger/common.py

@@ -1,5 +1,28 @@
+from datetime import timedelta
+
+from . import host
+
 LOG = None
 
+class YangDate:
+    def __init__(self, dt=None):
+        self.dt = dt if dt else host.HOST.now()
+
+    def __str__(self):
+        datestr = self.dt.strftime("%Y-%m-%dT%H:%M:%S%z")
+
+        # Translate strftime's timezone format (HHMM) to the one
+        # expected by YANG (HH:MM)
+        return datestr[:-2] + ':' + datestr[-2:]
+
+    @classmethod
+    def from_delta(cls, delta):
+        return cls(host.HOST.now() - delta)
+
+    @classmethod
+    def from_seconds(cls, seconds):
+        return cls.from_delta(timedelta(seconds=seconds))
+
 
 def insert(obj, *path_and_value):
     """"This function inserts a value into a nested json object"""

src/statd/python/yanger/ietf_routing.py

@@ -1,10 +1,9 @@
 from datetime import timedelta
 from re import match
 
-from .common import insert
+from .common import insert, YangDate
 from .host import HOST
 
-
 def uptime2datetime(uptime):
     """
     Convert uptime to YANG format (YYYY-MM-DDTHH:MM:SS+00:00)
@@ -36,11 +35,7 @@ def uptime2datetime(uptime):
         h += days * 24
 
     uptime_delta = timedelta(hours=h, minutes=m, seconds=s)
-    current_time = HOST.now()
-    last_updated = current_time - uptime_delta
-    date_timestd = last_updated.strftime('%Y-%m-%dT%H:%M:%S%z')
-
-    return date_timestd[:-2] + ':' + date_timestd[-2:]
+    return str(YangDate.from_delta(uptime_delta))
 
 
 def add_protocol(routes, proto):