Use a simple for-loop. List comprehensions are used to create new lists, don't use them for side-effects.
it = iter(dct['time'] for dct in a)
tot = sum(it, next(it))
for dct in a:
dct['time'] = tot
Another way to sum the dates will be to use reduce()
(functools.reduce
in Python 3):
>>> dates = [dct['time'] for dct in a]
>>> reduce(datetime.datetime.__add__, dates)
datetime.datetime(2014, 4, 7, 16, 5, 55)