Finding the same value in diffrent sheets

Hello,

I have one workbook with different sheets naming jan,feb, march...etc
theese are montly timesheets. i need to find the last week total in same job number
lastweek means ( some times month starting on wednesday , thus,fri ets , in this case i need to take the other 2, 3 days from previous month) .so how can i find the same job number in diffrent sheet and last week total


Thanks for your help in advanced
usha