How do I subtract two dates in Django/Python?

Question

I'm working on a little fitness tracker in order to teach myself Django. I want to graph my weight over time, so I've decided to use the Python Google Charts Wrapper. Google charts require that you convert your date into a x coordinate. To do this I want to take the number of days in my dataset by subtracting the first weigh-in from the last weigh-in and then using that to figure out the x coords (for example, I could 100 by the result and increment the x coord by the resulting number for each y coord.)

Anyway, I need to figure out how to subtract Django datetime objects from one another and so far, I am striking out on both google and here at the stack. I know PHP, but have never gotten a handle on OO programming, so please excuse my ignorance. Here is what my models look like:

class Goal(models.Model):     goal_weight = models.DecimalField("Goal Weight",          max_digits=4,          decimal_places=1)     target_date = models.DateTimeField("Target Date to Reach Goal")     set_date = models.DateTimeField("When did you set your goal?")     comments = models.TextField(blank=True)      def __unicode__(self):         return unicode(self.goal_weight)  class Weight(models.Model):     """ Weight at a given date and time. """      goal = models.ForeignKey(Goal)     weight = models.DecimalField("Current Weight",         max_digits=4,          decimal_places=1)     weigh_date = models.DateTimeField("Date of Weigh-In")     comments = models.TextField(blank=True)      def __unicode__(self):         return unicode(self.weight)      def recorded_today(self):         return self.date.date() == datetime.date.today() 

Any ideas on how to proceed in the view? Thanks so much!

Answer 1

You can just subtract the dates directly, which will yield a datetime.timedelta object:

dt = weight_now.weight_date - weight_then.weight_date 

A timedelta object has fields for days, seconds, and microseconds. From there, you can just do the appropriate math. For example:

hours = dt.seconds / 60 / 60    # Returns number of hours between dates weeks = dt.days / 7             # number of weeks between dates 

Answer 2

Django datetime objects are just regular Python datetime objects. When you subtract one datetime from another you get a timedelta object.

If you are looking to subtract a length of time from a datetime you need to subtract a timedelta object from it. For example:

>>> from datetime import datetime, timedelta >>> now = datetime.now() >>> print now 2010-05-18 23:16:24.770533 >>> this_time_yesterday = now - timedelta(hours=24) >>> print this_time_yesterday 2010-05-17 23:16:24.770533 >>> (now - this_time_yesterday).days 1