Time Conversions with SPICE
JPL's SPICE Toolkit (SPICELIB) is the premier software library for computations related to solar system geometry. It is freely distributed, and is also one of the best-documented libraries I have ever come across. SPICELIB also includes a comprehensive set of routines for date and time conversions. An example is shown here:
program spice_test
use iso_fortran_env, only: wp => real64
implicit none
interface
! SPICELIB routines
subroutine timout ( et, pictur, output )
import :: wp
implicit none
real(wp),intent(in) :: et
character(len=*),intent(in) :: pictur
character(len=*),intent(out) :: output
end subroutine timout
subroutine str2et ( string, et )
import :: wp
implicit none
character(len=*),intent(in) :: string
real(wp),intent(out) :: et
end subroutine str2et
subroutine furnsh ( file )
implicit none
character(len=*),intent(in) :: file
end subroutine furnsh
end interface
character(len=*),parameter :: time_in = &
'2017 Aug 12 00:00:00 TDB'
character(len=*),parameter :: pictur = &
'Mon DD,YYYY HR:MN:SC.#### UTC ::UTC'
real(wp) :: et
character(len=100) :: time_out
! load the leap second kernel:
call furnsh('naif0012.tls')
! example conversion:
call str2et(time_in, et)
call timout(et, pictur, time_out)
write(*,*) 'time_in: ', time_in
write(*,*) 'et: ', et
write(*,*) 'time_out: ', time_out
end program spice_test
A few things to note:
- Here we are using the SPICE routines str2et and timout to convert a string from a TDB calendar date to ephemeris time and then to a UTC calendar date. These routines are very flexible and can convert a wide range of date formats. Other routines are available to do other transformations.
- The base time system of SPICE is Barycentric Dynamical Time (TDB). "Ephemeris time" is a count of TDB seconds since the J2000 epoch (Jan 1, 2000 12:00:00).
- We have to load the latest leap second kernel (naif0012.tls in this case), which is necessary to define UTC.
- The SPICE routines are not in a module (the code is Fortran 77), and so have no explicit interfaces. Thus it is good practice to specify them as I do here.
The output of this example is:
time_in: 2017 Aug 12 00:00:00 TDB
et: 555768000.00000000
time_out: Aug 11,2017 23:58:50.8169 UTC