The 4th line of datevec() syntax is:
[Y, M, D, H, MN, S] = datevec(...)
Where it's self-explanatory that
-1 11 22 18 0 0
Y M D H MN S
Now, the fact that you're doing SomeDate - DateInFuture, determines the negative sign.
Thus,
datevec(-38.25)
says which Year, Month, etc.. is that serial date.
For intuition, supose you are 3 days Before Christ. Datevec would say you're in the Year before BC (thus -1), in the 12th month, 22nd day etc... (assuming he was born the 25th Dec).
