# Time Traveller Maths

On a Wednesday afternoon, Marty McFly starts his DeLorean at 14:00 hours and travels 30 years in the past.

After spending 3 days 30 minutes, travels 60 years to the future where he stays for another 3 days and 30 minutes.

He sets his clock to 30 years in the past to 14:00 hours and safely makes it to the present.

Question : So what day of the week is it for him today and what is the total time Marty McFly was actually "away" Time Travelling?

Hint : No Tricks, simple straight forward maths. Will clear your time travel concepts. Disregard leap year, location, time zone changes.

• When you ask how long he spent time travelling, you don't mean how much time elapsed for him while travelling from the future to the past, but rather how much time has passed for him from when he left the present to when he returned? – GentlePurpleRain Oct 21 '15 at 17:32
• @GentlePurpleRain The "absolute" total time he spent travelling. – kanchirk Oct 21 '15 at 18:08
• There are many different possible interpretations for what you ask. Please can you clarify. Obviously if someone leaves the present and returns to the present then an observer who does not time travel will say the elapsed time is zero. The traveller will disappear and instantly reappear -- perhaps looking different. The whole thing is entirely subjective unless you define your terms very tightly and specifically. What is your frame of reference? – chasly from UK Oct 21 '15 at 18:26
• Does his machine account for leap years, or did he just set it to go back (365 * 30) days? Did he travel close to a daylight savings clock change date? What day of the week is it for him today? or the day he got back to the future? Why are you asking how long he spent time travelling when you've already specified Consider Zero seconds for Time Travel.? – CactusCake Oct 21 '15 at 18:44
• @JoeMalpass I wanted to mention that in the hint, disregard leap year and location (time zone) etc.. – kanchirk Oct 21 '15 at 18:49

The day of the week is:

Tuesday

Because:

If today is X and I go back in time Y years, then spend 3 days and go back to the future by Y years, then the day I end up at is X+3. It might be easier to comprehend if we think about it in only days. Let's say I start at day 10. I go back in time 5 days to day 5. I spend 1 day in the past so now I'm at day 6. Then I go 5 days back to the future and end up at day 11, which is 1 day from the present day I started at. Going through all the time travel Marty does, there is a total of 3+3=6 days passing, so he would end up 6 days from when he left, which is a Tuesday.

I'm not sure what you mean by how much time he spent time travelling so I'll give both interpretations:

The total time that passed for him was 6 days and 1 hour. Just simple addition of 3 days 30 minutes + 3 days 30 minutes.

and

The total time he was gone from the present is 6 days exactly. When he finally comes back to the present he goes back 30 years to the time 14:00. When he does this, he is a total of 30 years, 6 days, 1 hour from the time he originally left the present (as shown above). This means he is at a time of 15:00 (he left at 14:00, and is X days + 1 hour ahead) before he goes back to the present. By going back 30 years to 14:00, he actually goes back 30 years 1 hour. This results in his total time "gone" from the present at 6 days 1 hour - 1 hour = 6 days.

• You have nailed it, you got the day correct. The Total time spent away can be considered as "Travelling" time, so that should be enough. You may please edit the question in case it seems a bit ambigious. Am marking it as correct answer. – kanchirk Oct 21 '15 at 18:47
• It depends on the specific date you leave though. If you leave from a leap year, and the 6 days spent time traveling crosses the February/March boundary, you end up returning on Wednesday. Example: You leave on Wed. Feb 24, 2016. You arrive in the past on Mon. Feb 24 1986, spend 3 days, and leave Thur. Feb 27. You travel 60 years to the future, arriving on Tue. Feb 27, 2046. Another 3 days and you head home, departing on Fri. Mar 2. You arrive on Mar 2, 2016 - which is a Wednesday! – Irishpanda Oct 21 '15 at 19:10
• Presumably the same would happen if either the past or future year was a leap year and the 3 days spent in that year crossed the boundary. If the final day in the future happened to be Feb 29th, you may never arrive home at all! – Irishpanda Oct 21 '15 at 19:15
• Just saw the comment to ignore leap years. Ah well, never mind! – Irishpanda Oct 21 '15 at 19:20

You say, "Zero seconds for Time Travel."

You ask, "How long did he take Time Travelling?"