Concatenating fields into a new textbox

Can somebody walk me through the steps of poulating a field with data from other form fields such that

field1 = 02/22/2009
field2 = 03/22/2009

textbox.value = For the period [field1] to [field2]

The output should look like "For the period 02/22/2209 to 03/22/2009"

Thanks.