[ferret_users] Error while merging files using append

[Swetha sivakumar <swetha.s2060@xxxxxxxxx>]

Hello all

I have 16 nc files stored at 16 different directories. I want to merge them all together into a single nc file. Below is code and error faced

use "/file/2001/u/d_2001.nc"
.

.

.

use "/file/2016/u/d_2016.nc"

save/append/file=check.nc U[d=1]

save/append/file=check.nc U[d=2]

**TMAP ERR: error in line definition
             disordered output coordinate value:          25  Axis: TIME
LIST/FORMAT=CDF/append/file=check.nc U[d=2]
Command file, command group, or REPEAT execution aborted
Executed by script: *** GO mergefiles_u.jnl ***

All 16 files have same x and y axis. But t varies for some years.What could possibly be the reason for this error?

TIA

Swetha S