how to substract two datetime.time values in django template,and how to format a duration as hour,minutes

Question

In a django app ,I am sending a list of Entry objects to the template.Each Entry object has a start, end times which are datetime.time values(from TimeFields on the form).While listing the Entry objects,I need to show the duration for each entry.Putting a duration field in model seemed to be reduntant since ,start and end times were already there

model

class Entry(models.Model):
    title = models.CharField(unique=True,max_length=50)
    starttime=models.TimeField(null=True)
    endtime=models.TimeField(null=True)
...

template

{% for entry in object_list %}
<tr> 
  <td> {{entry.title}} </td>
  <td> {{entry.starttime}}</td>
  <td> {{entry.endtime}}</td>
  <td> want to show duration here </td>
{%endfor %}

1.Is there any filter which can take two datetime.time values and calculate the duration in seconds. ie,

given
 t1=datetime.time(2,30,50) and
 t2=datetime.time(3,00,50)
should show
30 minutes

2.Also,is there a filter,that can show a duration in given number of minutes as hour,minutes if the minutes value is greater than 60

ie,

if duration is 50 minutes ==> 50 minutes
if duration is 150 minutes ==> 2 hours,30 minutes

update

def diff_in_time(start,end):
    startdelta=datetime.timedelta(hours=start.hour,minutes=start.minute,seconds=start.second)
    enddelta=datetime.timedelta(hours=end.hour,minutes=end.minute,seconds=end.second)
    return (enddelta-startdelta).seconds/60

when i tried with some sample time values ,it gave me the expected result

#start 11:35:00 pm
#end   00:05:00 am
start= datetime.time(23,35,00)
end = datetime.time(00,05,00)
print diff_in_time(start,end)

==> 30 minutes

#start 00:35:00 am
#end   01:35:00 am
start= datetime.time(00,35,00)
end = datetime.time(01,35,00)
print diff_in_time(start,end)

==>60 minutes

Answer 1

You've got a problem. You can't -- and shouldn't be able to -- compare two times. Is 11pm before or after 1am? It depends whether or not they're on the same day.

You need to either store them as datetime or something else that represents a relatively absolute time, or you need to turn them into datetimes like this:

def todatetime(time):
    return datetime.datetime.today().replace(hour=time.hour, minute=time.minute, second=time.second, 
                                             microsecond=time.microsecond, tzinfo=time.tzinfo)

def timestodelta(starttime, endtime):
    return todatetime(endtime) - todatetime(starttime)

This will fail to give the expected answer if the two calls to today span midnight.

Then you should probably use this app for a DurationField which stores a timedelta to store the result in the database for easy display.