set counter 0
set counterID -1
label .l -textvariable counter
frame .f
button .f.start -text "Start!" -command countUp
button .f.stop -text "Stop!" -command stopCounting
pack .f.start .f.stop -side left -expand y -fill both
pack .l -side top -fill x
pack .f -side top -fill both -expand y

proc countUp {} {
    global counterID counter

    incr counter
    set counterID [after 1000 countUp]
}
proc stopCounting {} {
    global counterID

    after cancel $counterID
}