# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""Time related utility functions."""
import numpy as np
from astropy.time import Time, TimeDelta
__all__ = [
"time_ref_from_dict",
"time_ref_to_dict",
"time_relative_to_ref",
"absolute_time",
]
# TODO: implement and document this properly.
# see https://github.com/gammapy/gammapy/issues/284
TIME_REF_FERMI = Time("2001-01-01T00:00:00")
[docs]def time_ref_from_dict(meta, format="mjd", scale="tt"):
"""Calculate the time reference from metadata.
Parameters
----------
meta : dict
FITS time standard header info
Returns
-------
time : `~astropy.time.Time`
Time object with ``format='MJD'``
"""
# Note: the float call here is to make sure we use 64-bit
mjd = float(meta["MJDREFI"]) + float(meta["MJDREFF"])
scale = meta.get("TIMESYS", scale).lower()
return Time(mjd, format=format, scale=scale)
[docs]def time_ref_to_dict(time, scale="tt"):
"""TODO: document and test.
Parameters
----------
time : `~astropy.time.Time`
Time object with ``format='MJD'``
Returns
-------
meta : dict
FITS time standard header info
"""
mjd = Time(time, scale=scale).mjd
i = np.floor(mjd).astype(np.int64)
f = mjd - i
return dict(MJDREFI=i, MJDREFF=f, TIMESYS=scale)
[docs]def time_relative_to_ref(time, meta):
"""Convert a time using an existing reference.
The time reference is built as MJDREFI + MJDREFF in units of MJD.
The time will be converted to seconds after the reference.
Parameters
----------
time : `~astropy.time.Time`
time to be converted
meta : dict
dictionary with the keywords ``MJDREFI`` and ``MJDREFF``
Returns
-------
time_delta : `~astropy.time.TimeDelta`
time in seconds after the reference
"""
time_ref = time_ref_from_dict(meta)
return TimeDelta(time - time_ref, format="sec")
[docs]def absolute_time(time_delta, meta):
"""Convert a MET into human readable date and time.
Parameters
----------
time_delta : `~astropy.time.TimeDelta`
time in seconds after the MET reference
meta : dict
dictionary with the keywords ``MJDREFI`` and ``MJDREFF``
Returns
-------
time : `~astropy.time.Time`
absolute time with ``format='ISOT'`` and ``scale='UTC'``
"""
time = time_ref_from_dict(meta) + time_delta
return Time(time.utc.isot)