生成标准格式
startTime = cd_inv_calendar(1958,01,1,00,00,00,"months since 1958-1-1",0)
endTime = cd_inv_calendar(2014,12,1,00,00,00,"months since 1958-1-1",0)
time = fspan(startTime, endTime, 684)
time!0 = "time"
time&time = time
time@units = "months since 1958-1-1"
ntim = dimsizes(time)
reference:http://www.ncl.ucar.edu/Document/Functions/date.shtml