You can use the delta value, make a variable called time and set it to 0, next in the func _process(delta) call time += delta, also you want dont want decimals so use the int method
var time = 0
func _process(delta):
time += delta
print(int(time))
And that is basically it, the time will move up, the print is just for visualization.
Also if you want to do it as score you can do
score = time
if alive:
time += delta
else:
time = time