CDO

try selecting a particular timestep/date, reset the time, and then merge to the original file; like

cdo -f nc -b F64 -mergetime -setrtomiss,-9e31,9e31 -settime,21:00:00 -seldate,2042-08-01T18:00:00 ifile ifile ofile

Also, I think it is better to set all the values of that particular timestep to "missing/_FillValue" unless you really want to change to something else

the values of this time step are not missing.
I just want to define correctly "Times"
I should have "2042-08-01_21:00:00", and I have " ".

sorry, I thought that particular timestep is missing somehow

try using settaxis operator to fix the time axis

I have tried:
cdo settaxis,2042-07-30,00:00:00,3hour ifile ofile
but I get
Times = 20420730, 20420730.125, 20420730.25, 20420730.375, 20420730.5,
20420730.625, 20420730.75, 20420730.875, 20420731, 20420731.125,
20420731.25, 20420731.375, 20420731.5, 20420731.625, 20420731.75,
20420731.875, 20420801, 20420801.125, 20420801.25, 20420801.375,
20420801.5, 20420801.625, 20420801.75, 20420801.875, 20420802,
20420802.125, 20420802.25, 20420802.375, 20420802.5, 20420802.625,
20420802.75, 20420802.875, 20420803, 20420803.125, 20420803.25,
20420803.375, 20420803.5, 20420803.625, 20420803.75, 20420803.875,
20420804 ;

It is NetCDF way [CF convention]of storing the time info. I do not think that is wrong. Please have a look at the section 25 Absolute time in this [I couldn't find an updated document] http://www-pcmdi.llnl.gov/drach/GDT_convention.html

If you want to check the time stamps of your file, use: sinfov or showtimestamp operators or

cdo outputtab,date,time ifile

Thanks.
But for some reason, when I use settaxis, it also changes all the dimensions name, and
my variable Times become a dimension ...