"Time till" program

I have created a program which works out days, hours, minutes, seconds and milliseconds until a date which the user inputs. Is it possible to shorten/make the output a little clearer?

from datetime import datetime

while True:
    inp = input("Enter date in format yyyy/mm/dd hh:mm:ss")
    try:
        then = datetime.strptime(inp, "%Y/%m/%d %H:%M:%S")
        break
    except ValueError:
        print("Invalid input")

now = datetime.now()
diff = then - now
print(diff, "until", inp)
print(diff.total_seconds(),"seconds")

Here you going to get two different answers in one for your question. One on the output issue, and one your coding. Let's start with the coding part.

I do like that keep trying until you get a proper input, however there is no way to exit this loop besides providing a legal date. This shouldn't be a problem as you suggest the valid format, but yet again it could be bad.

Even for small scripts I would get into the habit of using functions, and avoid code at the top level. Not only can that focus your coding, but it could also for you function to be called as part of a module, and allow for testing using modules like doctest. One common idiom to use is the if __name__ == '__main__': which then can call your function.

Using it as function could also allow for looping your script if you want to check multiple days, or allow dates to input from the command line using argument parsing.

Your names are not terrible, but they are not entirely good either. The now also kind of hides the datetime.now(). It's legal, and your code does work, but having variables the same as functions is a little unfortunate.

Regarding simplifying output

I expect that you would want output similar to "2 years, 4 months, 7 days, 8 hours, 40 minutes and 4 seconds". This however is harder than you think, as get the correct amount of months and years is tricky. I.e. How many days and months are there between 2015-02-03 and 2015-03-04? Is it "1 month and 1 day" or is it "29 days"? The latter would be correct if the month always have 30 days, and the first is the more conventional answer based on human logic.

The same kind of logic applies to years, where you'll possibly get the output wrong according to human logic. And with leap years the equation gets even worse. Both for months and days difference calculation.

As such you might want to evaluate what you actually want to output, and how to achieve this. One road could be to do it the hard way, and that is compare the dates part by part, but this will expand your code quite a bit.

I.e. when comparing the 2015-11-16 with 2016-03-14, you could try with "1 year" but as the to month is lower than from month, you have to subtract one and add 12 months to the difference. For months you'll then get "3 - 11 + 12" making 4 months. But yet again you'll have an issue since the 14th is before the 16th...

And this time you have to take into account the days of the previous month, February of 2016, which has 29 days, making the correct number of days: "14 - 16 + 29 = 27", and correct the month number to 3 months.

So the final difference is to be: 3 months and 27 days. (That is if I'm not mistaken, and done something wrong in my calculations.)

StackExchange Code Review Q#114213, answer score: 7