Given a datetime plot, how to i insert a text in the plot?

Hi I have a datetime plot. I want to insert a text in a specific part of the plot. Since my xaxis is no longer an integer, I am getting error while using "text".

How can i solve it?

x_p=time_select_20000(out_select(i,2):out_select(i,3));% datetime array
y_p=Alt_select_20000(out_select(i,2):out_select(i,3));%integer
plot(x_p,y_p)
i=1;
a=time_select_20000(out_select(i,2));%datetime point
b=Alt_select_20000(out_select(i,2));%integer point
n=10
str1 = sprintf('Flight level : %d',n)
text(a,b,str1)

I am getting this error

 Error using text
First two or three arguments must be numeric doubles.

How do i change the datetime to satisfy this?

[Stephen23]

This question is a continuation of this one:

http://www.mathworks.com/matlabcentral/answers/225993-how-to-write-a-value-on-a-specific-portion-which-is-constant-on-a-graph