I am disappoint, Wolfram Alpha
wolframalpha.com
wolframalpha.com
What is changing is your time zone from, say, GMT to BST.
This time zone change is listed there. It says 1:00pm EST to 1:00pm EDT on the page linked, and so it is incorrect.
http://www.wolframalpha.com/input/?i=hours+between+1pm+est+m...
Edit: Now I see that the link has both times in EST, which is not on point. See chrisrhoden's link in a sibling to this comment.
Ergo you can't search;
"hours between 1pm GMT march 10 and 1pm BST march 11 2012" http://www.wolframalpha.com/input/?i=hours+between+1pm+GMT+m...
And get a desired response. In fact it really screws up. Going to keep trying, also I thought that BST kicked in at the end of March. (Google informs me: 25th march)
0.9583 days = 22 hours 59 minutes 57 seconds
the time change does not apply everywhere, in places near the equator there is no daylight saving time change.
http://www.wolframalpha.com/input/?i=Hours+between+1pm+EST+M...
It also gives 0.9583 (23/24) as the amount of days, so one hand doesn't know what the other is doing.