right, I managed to get it working because of the code you send me. thanks a lot.
this is the final flow:
[{"id":"92124d52.6defa","type":"ui_button","z":"455ed50a.5deadc","name":"+","group":"3f0457bf.87fa48","order":1,"width":4,"height":2,"passthru":true,"label":"","tooltip":"","color":"","bgcolor":"","className":"","icon":"expand_less","payload":"1","payloadType":"num","topic":"Topic","topicType":"msg","x":870,"y":740,"wires":[["10304b9b.a0dda4"]]},{"id":"841e1d5f.13b9f","type":"ui_button","z":"455ed50a.5deadc","name":"-","group":"3f0457bf.87fa48","order":3,"width":4,"height":2,"passthru":true,"label":"","tooltip":"","color":"","bgcolor":"","className":"","icon":"expand_more","payload":"-1","payloadType":"num","topic":"Topic","topicType":"msg","x":870,"y":780,"wires":[["10304b9b.a0dda4"]]},{"id":"5976be9f.9c54a","type":"ui_button","z":"455ed50a.5deadc","name":"reset","group":"3f0457bf.87fa48","order":3,"width":4,"height":"1","passthru":true,"label":"reset","tooltip":"","color":"","bgcolor":"red","className":"","icon":"","payload":"true","payloadType":"bool","topic":"Topic","topicType":"msg","x":870,"y":820,"wires":[["b91031b3.bfdc8"]]},{"id":"10304b9b.a0dda4","type":"function","z":"455ed50a.5deadc","name":"","func":"let count = flow.get('count') || 0 // initialise to 0 if undefined\nmsg.payload += count\nflow.set('count', msg.payload)\nreturn msg","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":1070,"y":760,"wires":[["6bd4c04f.b2352","cc79f07f.6231d"]]},{"id":"6bd4c04f.b2352","type":"debug","z":"455ed50a.5deadc","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":1250,"y":800,"wires":[]},{"id":"b91031b3.bfdc8","type":"change","z":"455ed50a.5deadc","name":"to 0","rules":[{"t":"set","p":"count","pt":"flow","to":"-1","tot":"num"}],"action":"","property":"","from":"","to":"","reg":false,"x":990,"y":820,"wires":[["10304b9b.a0dda4"]]},{"id":"cc79f07f.6231d","type":"ui_text","z":"455ed50a.5deadc","group":"3f0457bf.87fa48","order":2,"width":0,"height":0,"name":"text","label":"uur:","format":"{{msg.payload}}","layout":"row-center","className":"","x":1230,"y":760,"wires":[]},{"id":"3f0457bf.87fa48","type":"ui_group","name":"Tijdklok","tab":"4ac8d4a5.3b285c","order":3,"disp":true,"width":4,"collapse":false,"className":""},{"id":"4ac8d4a5.3b285c","type":"ui_tab","name":"Controle","icon":"report","order":1,"disabled":false,"hidden":false}]