Times in XL are stored as Fractional days (i.e., 03:00 = 0.125). So
you can continue to add times >9999:59, but you can't display them
in time format. You can get a decimal format by multiplying by 24,
e.g.:
A1: 9999:30
A2: 1:45
A3: =(A1 + A2) * 24 ===> 10001.25
Or you could display the colon format as text (though you won't be
able to use it as a number):
A3: =INT((A1+A2)*24) & right(text(mod(A1+A2,1),"h:mm"),3)