Input the gui dates in whatever format you want as the user interface. To find the data in the spreadsheet and make the plots and markers, convert to Matlab serial date numbers via datenum and search on that. Unless the spreadsheet is truly tremendously large, you'll likely have better performance by just reading the whole thing into memory and then selecting the subset(s) of interest.
Depending on how the date fields are stored in Excel you may need to use the conversion to/from the initial dates between Matlab and Excel for their serial dates as shown in the Matlab topic at
See
doc datenum % and friends for Matlab serial date numbers and related functions
doc datetick % formatting plot axes with datenums
I'll note this is a good candidate for my "syntactic sugar" helper function iswithin to clean up top-level coding. Say your user has selected/input the two start and end dates and you've gotten the two values as
dn1=datenum(y1,m1,d1,h1,m1,s1);
dn2=datenum(y2,m2,d2,h2,m2,s2);
where the inputs are the beginning and end times, respectively. Then having read in the Excel file, and presuming you have a date string as the first column,
dn=datenum(x(:,1)); % the datenums of the Excel sheet
idx=iswithin(dn,dn1,dn2);
x=x(idx,:); dn=dn(idx); % save only the rows within the bounds
Now you can plot as
plot(dn,x(:,selectedcolmns);
datetick('x',... % set the axis to date; options as desired
In the above the function is
function flg=iswithin(x,lo,hi)
% returns T for values within range of input
% SYNTAX:
% [log] = iswithin(x,lo,hi)
% returns T for x between lo and hi values, inclusive
flg= (x>=lo) & (x<=hi);
Copy the above and place it in an m-file by the name iswithin.m and put it in some directory on the matlabpath so it will be found. I have a directory in my installation I've called "utilities" that contains this and a bunch of similar goodies.
