set counter 0
set counterID -1
label .l -textvariable counter
button .b -command startCount -text "Start!"
pack .l -fill x
pack .b -fill both -expand y

proc startCount {} {
    countUp
    .b configure -command stopCount -text "Stop!"
}
proc stopCount {} {
    global counterID

    after cancel $counterID
    .b configure -command startCount -text "Start!"
}
proc countUp {} {
    global counterID counter

    incr counter
    set counterID [after 1000 countUp]
}