Is there a way we can use a function like .tostartofweek() in formulas.
In short if the user enters date as 02/26/2013 i want the formula to get the entry_date.tostartofweek() in formula field
Any pointers would help
Nothing simple exists that I'm aware of.
However, check out the useful advanced formulae page: http://login.salesforce.com/help/doc/en/useful_advanced_formulas.htm
In particular, this has a formula for calculating the day of the week as a number (0 = Sunday...6=Sunday).
MOD(entry_date__c - DATE(1900, 1, 7), 7)
The above will give you Entry Date's day of the week. To get Sunday, you should be able to subtract the result of that from the Entry Date
DATE( YEAR(Entry_Date__c), Month(Entry_Date__c), DAY(Entry_Date__c)-MOD(entry_date__c - DATE(1900, 1, 7), 7))
Though you would need to add some complexity to this to handle the start of a month/year.
Edit:As @TomLogic points out below, you may be able to avoid using the DATE function and having to add complexity to handle start of month/year issues by subtracting the result of the Mod operation from the Date directly (though I've not yet tested it!)
Entry_Date__c - MOD(entry_date__c - DATE(1900, 1, 7), 7))
You might be able to start with the answers to this question on how to determine the day of week for a given date. At that point, it should be easy to subtract days to get back to Sunday or Monday (depending on when you think a week begins).