I will take an example of searching files added in last 7 days in 'document library.

 using (SPSite site = new Site("<site_url">)

{
using (SPWeb web = site.AllWebs["/"])      //Best practice to use SPWeb in using.
{

    DataTable dt;
    SPSiteDataQuery Query = new SPSiteDataQuery();      //Instantiate SPSiteDataQuery

    //Date value in CAML query is accepted in a specific format. Below is the format accepted. 

    string str7DaysBackDateTime = (DateTime.Now.Add(new TimeSpan(-7, 0, 0, 0, 0))).ToString("yyyy-MM-ddThh:mm:ssZ");
    string strQuery = String.Format("<Where><Gt><FieldRef Name=\"Modified\" />"
                +"<Value Type=\"DateTime\">{0}</Value></Gt></Where>"
                +"<OrderBy><FieldRef Ascending=\"FALSE\" Name=\"Modified\"/></OrderBy>"
                , str7DaysBackDateTime);
    Query.Query = strQuery;                     //Assign CAML query(without <Query> tag.
    Query.RowLimit = 15;                           //Max Number of rows you want in the result.
    StringBuilder sb = new StringBuilder();
    sb.Append("<Lists>");
    foreach (SPList list in web.Lists)
    {
        if (list.BaseType == SPBaseType.DocumentLibrary)
        {
            sb.Append("<List ID=\"" + list.ID.ToString() + "\"/>");
        }
   }
   sb.Append("</Lists>");
   Query.Lists = sb.ToString();         //The lists on which you want your query to be run.
                   
   dt = web.GetSiteData(Query);

}

} 

I guess the above code is selft explanatory and is having enough comments also to explain the things.

akhilesh tiwari