<form name="doit">
<div>
<input type="checkbox" name="y1_1" value="12" onClick="timeleft(doit)"><label>Pierwsze Pole</label>

<input type="checkbox" name="y1_2" value="35" onClick="timeleft(doit)"><label>Drugie Pole</label>

<input type="submit" name="mui" value="go!">
</div>
</form>

function timeleft(doit)
  {
if (doit.y1_1.checked) {
   button = parseInt(doit.y1_1.value);}
}

[HTML] pobierz, plaintext 
<input type="checkbox" name="y1_2" value="35" onClick="timeleft(this);">
[HTML] pobierz, plaintext 

function timeleft(checkbox)
  {
if (checkbox.checked) {
   button = parseInt(checkbox.value);}
}