why does session timeout becomes 1840-12-31

It's the zero point for MUMPS based languages. It shows that way if your session has no timeout. From James Poitras on why:

"When I decided on specifications for the date routine, I remembered reading of the oldest (one of the oldest?) U.S. citizen, a Civil War veteran, who was 121 years old at the time. Since I wanted to be able to represent dates in a Julian-type form so that age could be easily calculated and to be able to represent any birth date in the numeric range selected, I decided that a starting date in the early 1840s would be ‘safe.’ Since my algorithm worked most logically when every fourth year was a leap year, the first year was taken as 1841. The zero point was then December 31, 1840."

Taken from http://www.faqs.org/faqs/m-technology-faq/part1/