CDO

I got these errors when I used this function: cdo mergetime -selday,1/31 -selmon,12 ifile ofile

*** NOTE: Axis definition error on axis: time
 *** NOTE: Bounds given describe non-contiguous axis cells
 *** NOTE: Using the LOWER bound specified for each axis cell

Details of the data is as follows:

dimensions:
    lon = 72 ;
    lat = 37 ;
    level = 2 ;
    time = UNLIMITED ; // (4949 currently)
    nb2 = 2 ;
variables:
    double lon(lon) ;
        lon:standard_name = "longitude" ;
        lon:long_name = "longitude" ;
        lon:units = "degrees_east" ;
        lon:axis = "X" ;
    double lat(lat) ;
        lat:standard_name = "latitude" ;
        lat:long_name = "latitude" ;
        lat:units = "degrees_north" ;
        lat:axis = "Y" ;
    double level(level) ;
        level:standard_name = "air_pressure" ;
        level:long_name = "pressure level" ;
        level:units = "mb" ;
        level:positive = "down" ;
        level:axis = "Z" ;
    double time(time) ;
        time:standard_name = "time" ;
        time:long_name = "time" ;
        time:bounds = "time_bnds" ;
        time:units = "days since 1955-01-01 00:00:00" ;
        time:calendar = "365_day" ;
    double time_bnds(time, nb2) ;
        time_bnds:units = "days since 1955-01-01 00:00:00" ;
        time_bnds:calendar = "365_day" ;

Any helps will be really appreciated.
Thanks