Processing of data is described in: 

Kent, E. C., N. A. Rayner, D. I. Berry, M. Saunby, B. I. Moat, J. J. Kennedy, and D. E. Parker (2013), Global analysis of night marine air temperature and its uncertainty since 1880: The HadNMAT2 data set, J. Geophys. Res. Atmos., 118, 1281–1298, doi:10.1002/jgrd.50152.

http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50152

Due to a processing error data are missing for March 1994. 

netcdf HadNMAT.2.0.0.0 {
dimensions:
        longitude = 72 ;
        latitude = 36 ;
        time = 1572 ;
        nv = 2 ;
variables:
        float longitude(longitude) ;
                longitude:units = "degrees_e" ;
                longitude:modulo = 360.f ;
                longitude:standard_name = "longitude" ;
                longitude:long_name = "longitude" ;
        float latitude(latitude) ;
                latitude:units = "degrees_n" ;
                latitude:standard_name = "latitude" ;
                latitude:long_name = "latitude" ;
        float time(time) ;
                time:units = "days since 1850-1-1 0:0:0" ;
                time:long_name = "time" ;
                time:bounds = "time_bnds" ;
                time:calendar = "gregorian" ;
        float time_bnds(time, nv) ;
        float night_marine_air_temperature(time, latitude, longitude) ;
                night_marine_air_temperature:units = "degC" ;
                night_marine_air_temperature:long_name = "HadNMAT2 night marine air temperature" ;
                night_marine_air_temperature:missing_value = -999.f ;
                night_marine_air_temperature:_FillValue = -999.f ;

// global attributes:
                :Conventions = "CF-1.6" ;
                :Reference = "Kent, EC, NA Rayner, DI Berry, M Saunby, BI Moat, JJ Kennedy and DE Parker, 2013: Global analysis of night marine air temperature and its uncertainty since 1880, the HadNMAT2 Dataset, J. Geophys. Res. Atmospheres, 118, 1281-1298, doi: 10.1002/jgrd.50152" ;
                :Title = "Monthly 5 degree Night Marine Air Temperature, HadNMAT2" ;
                :source = "HadNMAT.2.0.0.0" ;
                :history = "NetCDF file generated Fri Dec 19 09:32:00 2014" ;
                :supplementary_information = "As used in IPCC AR5, note that data for March 1994 are missing due to a processing error" ;