! compute dtm at start of month from the monthly averages
use dtm.nc
set axis/mod month
define axis/t=0.5:11.5:1/mod tstart
let dtm_start = dtm[gt=tstart]
list/x=-150/y=-65/l=1:13 dtm,dtm_start
save/file=dtm_start.nc/clob dtm_start