CDO

Ok. So, I have a grads file with daily data. Starts 1 Sep 2017. I use CDO to convert to netcdf. If I then use

cdo showtimestamp file.nc

I get the correct result for the first data point: 2017-09-01T03:00:00.

If I use ncdump to look at the .nc file, I get this:

variables:
double time(time) ;
time:standard_name = "time" ;
time:units = "hours since 1-1-1 00:00:00" ;
time:calendar = "standard" ;
time:axis = "T" ;

and the first t value is 17677779. But, when I do the math, this t value corresponds to 2017-09-03 03:00:00. And, if I use Matlab to open the .nc file and display the times, it confirms that the first time slice is 2017-09-03. I am super confused about this two day offset. CDO tells me that the first time value is Sep 1 (using showtimestamp), but other methods tell me the first time value is Sep 3. Thanks in advance for any tips. I am sure it is something simple and obvious that I am missing!

Dave.