If you don't need an actual date, just using what you have will work. If it's not sorted correctly, you can change that (right click).
If you wanted just the week then a calculated field: right(str([Date]),2) would give you that. Week 1 2012 would be combined with Week 1 2011, unless you have year separated out, or the original date.
Year would be left(str([Date]),4) if you wanted it
To add to what Alex suggested, you could even go further to convert to the full date values in the year you are referencing (if your users are like mine, they often complain that they don't want to do any "conversions" in their head of which dates they're looking at):
int(left(str([Date]),4)) // gives you the year as an integer
int(right(str([Date]),2)) // gives you the week as an integer
You can do date math with these values:
// gives the first day of the date in your date field (used in next step)
DATEADD('year', int(left(str([Date]),4)) ,#1/1/1900#)
DATEADD('week', int(right(str([Date]),2)) , DATEADD('year', int(left(str([Date]),4)) ,#1/1/1900#) )
// gives the day of the start of the week in the year you specified.