Time from astropy.time not precise

Hello,

I encounter difficulties with Time. I’m working on a package to perform photometry and occultation.

For this last case, data need times values accurately estimated. Of course, data coming from different camera will will have different time format in the header.

to manage this without passing long time to build a time parser, i decided to use Time object which do exactly what i need. The problem is, i dont arrive to make accurate conversion between different format using Time.

let’s do an exemple:

t1 = '2022-03-24T23:13:41.390999'
t1 = Time(t1, format = 'isot', precision = len(t1.split('.')[-1]))
t2 = t1.to_value('jd')
# result is 2459663.4678401737

now let’s do reverse

t2 = Time(t2, format = 'jd', precision = len(str(t2).split('.')[-1]))
t3 = t2.to_value('isot')
# result is 2022-03-24T23:13:41.0551352177

as you can see i don’t fall back on the same value and the difference is quite high. I would like to fall back on the original one.

thank you in advance

[maxnoe]

Probably we should keep this open to address the issue with precsion > 9 that @saimn found?

[mhvk]

As for how to fix this, right now we do check precision as a property, but not on input:

In [42]: t = Time('J2000')

In [43]: t = Time('J2000', precision=10)

In [44]: t.precision
Out[44]: 10

In [45]: t.precision = 10
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-45-59f84a57d617> in <module>
----> 1 t.precision = 10

/usr/lib/python3/dist-packages/astropy/time/core.py in precision(self, val)
    608         del self.cache
    609         if not isinstance(val, int) or val < 0 or val > 9:
--> 610             raise ValueError('precision attribute must be an int between '
    611                              '0 and 9')
    612         self._time.precision = val

ValueError: precision attribute must be an int between 0 and 9

Seems reasonable to check on input as well.