There are several issues going on here. First, a couple of typos. I believe you meant "Text" not "Texy" and "datestr" not "datastr".
Secondly, if you go to the link that is provided with the error message, you might notice this about "eval":
If possible, avoid using these functions altogether. See Alternatives to the eval Function.
What I believe is happening is that the timer function is called in its own workspace. In this case, you are providing it an anonymous function, so it is using the anonymous function's workspace. In this workspace, the variable "app" does not exist, so you wouldn't have access to the label anyway. Also, it seems that anonymous functions are given static workspaces, meaning you cannot assign new variables, which is what disallows it from creating a new struct called "app".
I would recommend making a new private function, called something like "setLabelToCurrentTime". Then you can just make the call to:
app.Label.Text = datestr(now);
within that function, and can just provide this to the timer object:
t.TimerFcn = @(~,~) setLabelToCurrentTime(app);