To reproduce:
- start calc (M-x calc)
- enter algebraic mode (')
- enter today's date as <2019-07-27> then hit RET
- convert to Julian day count (t J)
- notice that the result is 2458690, which is wrong (the correct value
 is 2458692)

Oddly enough, Emacs' calendar produces the correct value. To see this,
start the calendar (M-x calendar), navigate to the date above, then
press "p a". This displays the correct value:

Astronomical (Julian) day number (at noon UTC): 2458692.0

To cross-check, several web sites are available to compute the Julian
day number, e.g. http://numerical.recipes/julian.html


In GNU Emacs 26.2 (build 1, x86_64-apple-darwin18.2.0, NS appkit-1671.20 Version 10.14.3 (Build 18D109))
of 2019-04-13 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.1671
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
user-error: The mark is not set now, so there is no region
Welcome to the GNU Emacs Calculator! Press ‘?’ or ‘h’ for help, ‘q’ to quit
<help> DEL is undefined

Configured using:
'configure --with-ns '--enable-locallisppath=/Library/Application
Support/Emacs/${version}/site-lisp:/Library/Application
Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES THREADS

Important settings:
 value of $LANG: fr_CH.UTF-8
 locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
 tooltip-mode: t
 global-eldoc-mode: t
 eldoc-mode: t
 electric-indent-mode: t
 mouse-wheel-mode: t
 tool-bar-mode: t
 menu-bar-mode: t
 file-name-shadow-mode: t
 global-font-lock-mode: t
 font-lock-mode: t
 blink-cursor-mode: t
 auto-composition-mode: t
 auto-encryption-mode: t
 auto-compression-mode: t
 line-number-mode: t
 transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml mml-sec password-cache epa derived epg epg-config
gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils calccomp calc-arith calc-alg
calc-misc calc-vec calc-forms calc-ext calc-aent calc-menu easymenu calc
calc-loaddefs calc-macs elec-pair time-date tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/ns-win ns-win
ucs-normalize mule-util term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads kqueue cocoa ns multi-tty make-network-process emacs)

Memory information:
((conses 16 238377 12447)
(symbols 48 23518 1)
(miscs 40 46 241)
(strings 32 36281 1818)
(string-bytes 1 950248)
(vectors 16 37616)
(vector-slots 8 761908 14102)
(floats 8 51 104)
(intervals 56 230 0)
(buffers 992 13))