time = "01:30" # 1 minute and 30 seconds
time2 = time + 10 seconds

def format_duration(seconds):
    minutes, seconds = divmod(seconds, 60)
    hours, minutes = divmod(minutes, 60)
    return '{:02d}:{:02d}:{:02d}'.format(hours, minutes, seconds)

>>> def format_duration(seconds):
...     minutes, seconds = divmod(seconds, 60)
...     hours, minutes = divmod(minutes, 60)
...     return '{:02d}:{:02d}:{:02d}'.format(hours, minutes, seconds)
...
>>> duration = 170
>>> format_duration(duration)
'00:02:50'
>>> duration += 10
>>> format_duration(duration)
'00:03:00'