For this example, with the defined names as shown
DaT DoM DoS
12/28/2006 1 59
12/29/2006 2 48
12/30/2006 0 26
12/31/2006 4 55
1/1/2007 5 23
1/2/2007 6 61
=SUMPRODUCT((DaT<DATE(2007,1,1))*DoM)/
SUMPRODUCT((DaT<DATE(2007,1,1))*(DoM0)*DoS)
=4.32%
Your SUMIF formula would give the wrong answer of 3.72%