Got it.
Using
msg.template:
You can also define the template content viamsg.template, so you can use external files for example.
Template will be reloaded on input if it has changed.
As msg.template didn't change, nothing happens, I have added some code to change the iframe each time the inject node is triggered. This works, or at least I see the iframe reloading.
[{"id":"71cda6b.e5d0fd8","type":"inject","z":"7a62b435.e5dde4","name":"","topic":"","payload":"","payloadType":"date","repeat":"60","crontab":"","once":true,"onceDelay":0.1,"x":334,"y":192,"wires":[["98f7b0fe.6741e"]]},{"id":"b1ef7bc9.41b2","type":"debug","z":"7a62b435.e5dde4","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":770,"y":192,"wires":[]},{"id":"82d934f6.ddccc8","type":"ui_template","z":"7a62b435.e5dde4","group":"2f5bb252.c9d756","name":"Buienradar","order":1,"width":"5","height":"5","format":"","storeOutMessages":true,"fwdInMessages":true,"templateScope":"local","x":622,"y":192,"wires":[["b1ef7bc9.41b2"]]},{"id":"98f7b0fe.6741e","type":"function","z":"7a62b435.e5dde4","name":"","func":"d = new Date().getTime()\no = \"<IFRAME SRC='https://api.buienradar.nl/image/1.0/RadarMapNL?w=256&h=256&d=\"+d+\"' NORESIZE SCROLLING=NO HSPACE=0 VSPACE=0 FRAMEBORDER=0 MARGINHEIGHT=0 MARGINWIDTH=0 WIDTH='100%' HEIGHT='100%'></IFRAME>\"\nreturn {template:o};","outputs":1,"noerr":0,"x":482,"y":192,"wires":[["82d934f6.ddccc8"]]},{"id":"2f5bb252.c9d756","type":"ui_group","z":"","name":" Huidig","tab":"61b227eb.4f6b9","order":1,"disp":false,"width":"14","collapse":false},{"id":"61b227eb.4f6b9","type":"ui_tab","z":"","name":"Weer","icon":"dashboard","disabled":false,"hidden":false}]