I am not certain what you are asking.
If you want to turn the first 3 variables into a dateime variable, this works —
LD = load('DATA.mat');
c = struct2cell(LD);
T1 = c{:};
T1.YMD = datetime(T1.Year,T1.Month,T1.Days);
First10Rows = T1(1:5,:)
producing —
First10Rows =
5×20 table
Year Month Days SMB_larsenmm TP_larsenmm SF_larsenmm ES_larsenmm SMB_priestleymm TP_priestleymm SF_priestleymm ES_priestleymm SMB_talosmm TP_talosmm SF_talosmm ES_talosmm SMB_mpmm TP_mpmm SF_mpmm ES_mp_mmi YMD
____ _____ ____ ____________ ___________ ___________ ___________ _______________ ______________ ______________ ______________ ___________ __________ __________ __________ __________ _________ _________ __________ ___________
1998 1 1 -0.1223 0.15577 0.1562 -0.27807 -0.18414 2.8e-06 0.0015335 -0.18414 0.015194 0.018041 0.018084 -0.0028467 -0.002903 2.8e-06 0.0001269 -0.0029058 01-Jan-1998
1998 1 2 -0.38258 2.8e-06 2.8e-06 -0.38258 -0.27416 0.0004992 0.0001269 -0.27466 0.076073 0.068516 0.071046 0.0075563 0.000926 2.8e-06 0.0006234 0.0009232 02-Jan-1998
1998 1 3 -0.2277 0.11163 0.11329 -0.33933 -0.049567 0.17025 0.17076 -0.21982 0.87941 0.88927 0.88977 -0.0098669 -0.0053178 2.8e-06 0.0006647 -0.0053206 03-Jan-1998
1998 1 4 -0.36473 0.0014508 0.002361 -0.36618 -0.19776 0.019076 0.016429 -0.21684 0.16956 0.18026 0.18086 -0.0107 -0.0002722 2.8e-06 0.0001269 -0.000275 04-Jan-1998
1998 1 5 -0.36937 2.8e-06 2.8e-06 -0.36937 -0.21912 2.8e-06 2.8e-06 -0.21912 -0.0068196 2.8e-06 0.0006232 -0.0068224 0.0018871 0.0004992 0.0007888 0.0013879 05-Jan-1998
The ‘YMD’ variable is concatenated to the end of the existing table as the 
variable. .
variable. . .
