You could do this:
In [1]: import datetime as DT
In [2]: d = DT.datetime(1900,1,1,2,0)
In [3]: m = DT.datetime(1900,1,1,6,0)
In [4]: delta = d - DT.datetime(1900,1,1,0,0)
In [5]: m + delta
Out[5]: datetime.datetime(1900, 1, 1, 8, 0)
But note that this will give you a different answer if the year/month/date of d
are different than 1900/1/1.