Changing the default datacursor settings

How might I change the default datacursor settings for figures so that the x position number is shown in a date string format.

I have generated the following function which successfully changes the datacursor text, but not permanently i.e. I have to re-run this function each time I open my saved figures (saved as .fig).

function [txt] = plotDateTagfcn(obj,event_obj)

% Display 'date string for cursor x position' 

pos = get(event_obj,'Position');

txt = {['Date: ',datestr(pos(1))],['Magnitude: ',num2str(pos(2))]};

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%%I then  call this function by using:

set(figure_handle,'UpdateFcn',@plotDateTagfcn)

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I’d really appreciate some help on this.

Thanks

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RobbieH