i want to figure out the time from the beginning of the day given a days milliseconds.

so say i'm given this: 1340323100024 which is like mid day of 6/21/2012. now i want the milliseconds from the beginning of the day, which would be 1340262000000 (at least i think that's what it's supposed to be.)

how do i get 1340262000000 from 1340323100024?

i tried doing

Math.floor(1340323100024/86400000) * 86400000 

but that gives me 1340236800000, which if i create a date object out of it, says its the 20th.

i know i can create a date object from 1340323100024, then get the month, year, and date, to create a new object which would give me 1340262000000, but i find it ridiculous i can't figure out something so simple.

any help would be appreciated.

btw, i'm doing this in javascript if it makes any difference.