M=[1997 01 01 00 00 0.5
1997 01 01 00 15 0.2
1997 01 01 00 30 0.0
1997 01 01 00 45 0.0
1997 01 01 01 00 0.0
1997 01 01 01 15 0.0
1997 01 01 01 30 0.1
1997 01 01 01 45 0.3
1997 01 01 02 00 0.5]
[ii,jj,kk]=unique(M(:,1:4),'rows')
out=[ii accumarray(kk,M(:,6))]