importXML output is automatically updated (about every two hours), but you still need a script to save the past values somewhere: usually, on another sheet. Enter the following using Tools > Script Editor, adjusting the sheet names and the cell with the value of interest. Set it to run daily or weekly using Resources > Current project's triggers.
function storeValue() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheetByName('Sheet1'); // where importXML is
var value = sheet.getRange("B3").getValue(); // where the cell of interest is
var sheet2 = ss.getSheetByName('Sheet2'); // where to store the data
var height = sheet2.getLastRow();
sheet2.insertRowAfter(height);
sheet2.getRange(height+1, 1, 1, 2).setValues([[new Date(), value]]);
}
At each run, the script appends two cells at the bottom of Sheet2: one with the timestamp (should be formatted as such), one with the value corresponding to that timestamp.
A version for storing a range of values such as B3:B7:
function storeValue() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheetByName('Sheet1');
var values = sheet.getRange("B3:B7").getValues()[0];
var sheet2 = ss.getSheetByName('Sheet2'); // where to store the data
var height = sheet2.getLastRow();
sheet2.insertRowAfter(height);
sheet2.getRange(height+1, 1, 1, 6).setValues([[new Date()].concat(values)]);
// setting 6 cells instead of 2 on the previous line
}