No occurrence <> missing data

As I closed the merge request (cf #64 (closed)) having the relevant comments, and closed MRs are gone forever, I restate the reference to Jim's suggestion here

I went ahead and used 'days since YYYY-MM-DD 00:00:00' for my first and 
last frost dates, since they are valid dates. My files are structured as 
(example for first frost date):

    dimensions:
             time = UNLIMITED ; // (56 currently)
             lon = 960 ;
             lat = 490 ;
             bnds = 2 ;
    variables:
             double time(time) ;
                     time:standard_name = "time" ;
                     time:long_name = "time" ;
                     time:axis = "T" ;
                     time:units = "days since 1900-01-01 00:00:00" ;
                     time:calendar = "gregorian" ;
                     time:bounds = "time_bounds" ;
             double time_bounds(time, bnds) ;
             double lon(lon) ;
                     lon:standard_name = "longitude" ;
                     lon:long_name = "longitude" ;
                     lon:units = "degrees_east" ;
                     lon:modulo = 360. ;
                     lon:axis = "X" ;
                     lon:bounds = "lon_bounds" ;
             double lon_bounds(lon, bnds) ;
             double lat(lat) ;
                     lat:standard_name = "latitude" ;
                     lat:long_name = "latitude" ;
                     lat:units = "degrees_north" ;
                     lat:axis = "Y" ;
                     lat:bounds = "lat_bounds" ;
             double lat_bounds(lat, bnds) ;
             float first_freeze_date(time, lat, lon) ;
                     first_freeze_date:_FillValue = 1.e+20f ;
                     first_freeze_date:missing_value = 1.e+20f ;
                     first_freeze_date:comment = "Date of the first day  ...." ;
                     first_freeze_date:flag_meanings = "No_Freeze_Following" ;
                     first_freeze_date:long_name = "First freeze date" ;
                     first_freeze_date:valid_min = 0. ;
                     first_freeze_date:flag_values = -2. ;
                     first_freeze_date:units = "days since 1900-01-01 00:00:00" ;
                     first_freeze_date:calendar = "standard" ;
```