calculate the warranty duration from the dates provided

core/utils.py

@@ -0,0 +1,21 @@
+def td_format(td_object):
+    """A helper function to convert time deltas to a human readable format.
+       Taken from here."""
+    seconds = int(td_object.total_seconds())
+    periods = [
+        ('year',        60*60*24*365),
+        ('month',       60*60*24*30),
+        ('day',         60*60*24),
+        ('hour',        60*60),
+        ('minute',      60),
+        ('second',      1)
+    ]
+
+    strings = []
+    for period_name, period_seconds in periods:
+        if seconds >= period_seconds:
+            period_value, seconds = divmod(seconds, period_seconds)
+            has_s = 's' if period_value > 1 else ''
+            strings.append("%s %s%s" % (period_value, period_name, has_s))
+
+    return ", ".join(strings)

devices/models/warranty.py

@@ -1,6 +1,7 @@
 from django.db import models
 
 from core.models import Category
+from core.utils import td_format
 
 from .device import Device
 
@@ -16,7 +17,6 @@ class Warranty(models.Model):
     device = models.ForeignKey(Device, on_delete=models.CASCADE)
     valid_from = models.DateField()
     valid_until = models.DateField()
-    duration_in_years = models.IntegerField()
     warranty_type = models.ForeignKey(WarrantyType, models.SET_NULL,
                                       blank=True, null=True)
 
@@ -29,3 +29,9 @@ class Warranty(models.Model):
     @property
     def customer(self):
         return self.device.customer
+
+    @property
+    def duration_in_years(self):
+
+        delta = self.valid_until - self.valid_from
+        return td_format(delta)