Rather than trying to create a string containing a negative year to represent a BCE date and passing that string into datetime, I'd call datetime with three inputs containing the year, month, and day data. In the example below I'm also specifying the format with which to display the datetime, using G to display the era rather than displaying the year as a negative number.
y = -38;
m = 4;
d = 15;
April15BCE = datetime(y, m, d, Format="dd MMM y G")
A year value of 0 corresponds to 1 BCE, so April15BCE lists a year of 39 BCE for y = -38.
datetime(0, 4, 15, Format="dd MMM y G")
datetime(1, 4, 15, Format="dd MMM y G")
