Skyfield time arithmetic

Hi,

I am trying to compute B: skyfield.Date that is a minute before A: skyfield.Date.

How should I go about this?

I tried using datetime.timedelta to represent one minute and subtract it, but it is not supported.

from skyfield.api import load
from datetime import timedelta
ts = load.timescale()
A = ts.now()
B = A - datetime.timedelta(minutes=1) #TypeError: unsupported operand type(s) for -: 'Time' and 'datetime.timedelta'

I couldn’t find a good way to do this in the examples or in the source of skyfield.timelib (timelib.py). Any idea?

[brandon-rhodes]

Yes, you should get good results if you subtract 1.0 / 24 / 60 from the Terrestrial Time. Something like:

minute = 1.0 / 24 / 60
t2 = ts.tt_jd(t.tt - minute)

By the way, were the : characters in your sample code your own notation, or part of a framework you’re using? They didn’t work when I tried them in the version of Python on this laptop.

[LIII-XXII]

these are python 3 type declarations (available from python 3.5/3.6): https://docs.python.org/3/library/typing.html thanks for the hint. We can close this now.