[Advaita-l] Pradosham around Sunset time

[S Jayanarayanan]

What precise time duration or interval constitutes "Pradosham" with regards to sunset? Here are various websites giving quite different answers:
 

http://hindusphere.com/pradosham-2013-dates/
 
"Pradosh kaal is the time period of 2 hours 24 minutes after the sunset."
 
http://www.agasthiar.org/AUMzine/0001-timing1.htm
"Nitya Pradosham is one and a half hours before dusk and half an hour thereafter. Assuming a 6 p.m. sunset, Nithya Pradosham would be between 4:30 p.m. and 6:30 p.m. everyday."
http://www.astrologypredict.com/special-category.php?page=Removal+of+Bad+Karma
"All Pradoshams occur between 1-1/2 hours before and right up until the moment of sunset in the time zone where you are living physically."
http://www.drikpanchang.com/vrats/pradoshdates.html
"19 December (Friday) Pradosh Vrat (K) 16:47 to 19:42" (i.e.  ~ 3 hours!)
http://voices.yahoo.com/pradosham-important-saivite-worship-ritual-observed-7552206.html
"To be precise Pradosham is the specific time period of 48 minutes (24 minutes before sunset and 24 minutes after sun set) that occurs every day (Nithya Pradosham)."
http://angelray8.blogspot.com/2012/09/all-about-pradosham.html
 
"Pradosham literally means the removal of sins ( Pra - Removal , Dosham - Sin) is one of the most important  prayer  performed to Lord Shiva. It comes every Thiriyodasi tithi' (thirteenth lunar (moon) day of the waxing as well as waning moon phases) in all Shiva temples about 48 minutes during sun set, moon day and prayers period would be between 4.30 p.m till 6.30 p.m (‘a period just before sunset and after sunset’.)"
 
http://samtnewsletter.blogspot.com/2013/08/different-types-of-pradosham-and.html
 
"The daily time before 90 minutes of sunset is called as nithya pradosham."