num2date returns 01:00:00 or 00:59:59 depending on calendar

[demorym]

Good afternoon,

Here is an issue I posted on Unidata/netcdf4-python#810 two weeks ago, and I have been advised to submit it here. Here it is below:

You may already be aware of this problem, but I didn't find how to solve it.
I have an output with hourly intervals written in days since 1979-01-01 00:00:00.
The first time step is 0. The second is 0.041666666667.
I have noticed that num2date does not give the same results when reading the second time step depending on the calendar used. It sometimes give 1979-01-01 01:00:00 (which is what we want), and sometimes 1979-01-01 00:59:59 (see below).
How can we fix it so num2date would give the correct result (01:00:00. 02:00:00, etc.) whatever calendar is used?

Thank you for your help,
Marie-Estelle

time_in1=0.041666666667
print(time_in_units)
days since 1979-01-01 00:00:00
dt_in1 = num2date(time_in1,time_in_units,calendar='standard')
str(dt_in1)
'1979-01-01 01:00:00'
dt_in1 = num2date(time_in1,time_in_units,calendar='gregorian')
str(dt_in1)
'1979-01-01 01:00:00'
dt_in1 = num2date(time_in1,time_in_units,calendar='proleptic_gregorian')
str(dt_in1)
'1979-01-01 01:00:00'
dt_in1 = num2date(time_in1,time_in_units,calendar='noleap')
str(dt_in1)
'1979-01-01 00:59:59'
dt_in1 = num2date(time_in1,time_in_units,calendar='365_day')
str(dt_in1)
'1979-01-01 00:59:59'
dt_in1 = num2date(time_in1,time_in_units,calendar='360_day')
str(dt_in1)
'1979-01-01 00:59:59'
dt_in1 = num2date(time_in1,time_in_units,calendar='julian')
str(dt_in1)
'1979-01-01 01:00:00'
dt_in1 = num2date(time_in1,time_in_units,calendar='all_leap')
str(dt_in1)
'1979-01-01 00:59:59'
dt_in1 = num2date(time_in1,time_in_units,calendar='366_day')
str(dt_in1)
'1979-01-01 00:59:59'

[jswhit]

Looks like a roundoff error - the accuracy of the calculation is around a millisecond.

[demorym]

Do you know how I could go around this?

[jswhit]

No, not without a rewrite of our julian day arithmetic to preserve more precision.

[jswhit]

Don't know whether this helps, but if you know the precision is 1 hour, you can do

d.strftime(format="%Y-%m-%d %H")

edited: never mind, that doesn't help - you'll get a zero printed for the hour.

[jswhit]

Adding a small offset to the time seems to do it, i.e.

>> d = cftime.num2date((1./24.)+1.e-6,'days since 1979-01-01 00:00:00',calendar='365_day')
>> print d

1979-01-01 01:00:00

[demorym]

It works indeed. Thank you so much for your help!