Week : 7 (Sunday 07/02/2010 - Saturday 13/02/2010)
=CONCATENATE(INT(([Task Date]-DATE(YEAR([Task Date]),1,1)+(TEXT(WEEKDAY(DATE(YEAR([Task Date]),1,1)),"d")))/7)+1, " (", TEXT([Task Date]-WEEKDAY([Task Date])+1,"dddd"), " ", TEXT([Task Date]-WEEKDAY([Task Date])+1,"dd/mm/yyyy"), " - " , TEXT([Task Date]+7-WEEKDAY([Task Date]),"dddd"), " " , TEXT([Task Date]+7-WEEKDAY([Task Date]),"dd/mm/yyyy"), ")")

Week : 7
=INT(([Task Date]-DATE(YEAR([Task Date]),1,1)+(TEXT(WEEKDAY(DATE(YEAR([Task Date]),1,1)),"d")))/7)+1

Week : Sunday 07/02/2010
=TEXT([Task Date]-WEEKDAY([Task Date])+1,"dddd"), " ", TEXT([Task Date]-WEEKDAY([Task Date])+1,"dd/mm/yyyy")

Week : Saturday 13/02/2010
=TEXT([Task Date]+7-WEEKDAY([Task Date]),"dddd"), " ", TEXT([Task Date]+7-WEEKDAY([Task Date]),"dd/mm/yyyy")

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Common Date Time formulas for Sharepoint – Calculated Fields

Get Week of the year
=DATE(YEAR([Start Time]),MONTH([Start Time]),DAY([Start Time]))+0.5-WEEKDAY(DATE(YEAR([Start Time]),MONTH([Start Time]),DAY([Start Time])),2)+1

First day of the week for a given date:
=[Start Date]-WEEKDAY([Start Date])+1

Last day of the week for a given date:
=[End Date]+7-WEEKDAY([End Date])

First day of the month for a given date:
=DATEVALUE(“1/”&MONTH([Start Date])&”/”&YEAR([Start Date]))

Last day of the month for a given year (does not handle Feb 29). Result is in date format:
=DATEVALUE (CHOOSE(MONTH([End Date]),31,28,31,30,31,30,31,31,30,31,30,31) &”/” & MONTH([End Date])&”/”&YEAR([End Date]))

Day Name of the week : e.g Monday, Mon
=TEXT(WEEKDAY([Start Date]), “dddd”)
=TEXT(WEEKDAY([Start Date]), “ddd”)

The name of the month for a given date – numbered for sorting – e.g. 01. January:
=CHOOSE(MONTH([Date Created]),”01. January”, “02. February”, “03. March”, “04. April”, “05. May” , “06. June” , “07. July” , “08. August” , “09. September” , “10. October” , “11. November” , “12. December”)

Get Hours difference between two Date-Time :
=IF(NOT(ISBLANK([End Time])),([End Time]-[Start Time])*24,0)

Date Difference in days – Hours – Min format : e.g 4days 5hours 10min :
=YEAR(Today)-YEAR(Created)-IF(OR(MONTH(Today)<MONTH(Created),AND(MONTH(Today)=MONTH(Created), DAY(Today)<DAY(Created))),1,0)&” years, “&MONTH(Today)-MONTH(Created)+IF(AND(MONTH(Today)<=MONTH(Created),DAY(Today)<DAY(Created)),11,IF(AND(MONTH(Today)<MONTH(Created),DAY(Today)>=DAY(Created)),12,IF(AND(MONTH(Today)>MONTH(Created),DAY(Today)<DAY(Created)),-1)))&” months,“&Today-DATE(YEAR(Today),MONTH(Today)-IF(DAY(Today)<DAY(Created),1,0),DAY(Created))&” days”
OR
=IF(HOUR([Ending Date])>HOUR([Starting Date]),DATEDIF([Starting Date],[Ending Date],”d”)&” days “&HOUR([Ending Date]-[Starting Date])&” hours “,(DATEDIF([Starting Date],[Ending Date],”d”)-1)&” days “& HOUR([Ending Date]-[Starting Date])&” hours “)&MINUTE([Ending Date]-[Starting Date])&” minutes”

You can get Get more formulas from
http://office.microsoft.com/en-us/sharepointtechnology/HA011609471033.aspx

Reference

Taming the Elusive “Calculated Column” – Text and Data (Part V)