giuliayearlyDELTA2.Year is a char array, so giuliayearlyDELTA2.Year(1) is '0', and you're calling datetime on that value.
Some options (I didn't look at the rest of your code so not sure where you're headed):
Option 1, call datetime on the row of chars:
xstart = datetime(giuliayearlyDELTA2.Year(1,:))
Option 2, convert the table variable to datetime and then index the first item
xstart = giuliayearlyDELTA2.Year(1);
Option 3, convert the table variable to string, then call datetime on the first item:
xstart = datetime(giuliayearlyDELTA2.Year(1));