How can I produce a human readable difference when subtracting two UNIX timestamps using Python?

Question

This question is similar to this question about subtracting dates with Python, but not identical. I\'m not dealing with strings, I have to figure out the difference between

Answer 1

def humanize_time(amount, units = 'seconds'):    

    def process_time(amount, units):

        INTERVALS = [   1, 60, 
                        60*60, 
                        60*60*24, 
                        60*60*24*7, 
                        60*60*24*7*4, 
                        60*60*24*7*4*12, 
                        60*60*24*7*4*12*100,
                        60*60*24*7*4*12*100*10]
        NAMES = [('second', 'seconds'),
                 ('minute', 'minutes'),
                 ('hour', 'hours'),
                 ('day', 'days'),
                 ('week', 'weeks'),
                 ('month', 'months'),
                 ('year', 'years'),
                 ('century', 'centuries'),
                 ('millennium', 'millennia')]

        result = []

        unit = map(lambda a: a[1], NAMES).index(units)
        # Convert to seconds
        amount = amount * INTERVALS[unit]

        for i in range(len(NAMES)-1, -1, -1):
            a = amount // INTERVALS[i]
            if a > 0: 
                result.append( (a, NAMES[i][1 % a]) )
                amount -= a * INTERVALS[i]

        return result

    rd = process_time(int(amount), units)
    cont = 0
    for u in rd:
        if u[0] > 0:
            cont += 1

    buf = ''
    i = 0
    for u in rd:
        if u[0] > 0:
            buf += "%d %s" % (u[0], u[1])
            cont -= 1

        if i < (len(rd)-1):
            if cont > 1:
                buf += ", "
            else:
                buf += " and "

        i += 1

    return buf

Example of use:

>>> print humanize_time(234567890 - 123456789)
3 years, 9 months, 3 weeks, 5 days, 11 minutes and 41 seconds
>>> humanize_time(9, 'weeks')
2 months and 1 week

Advantage (You don't need third parties!).

Improved from "Liudmil Mitev" algorithm. (Thanks!)