A C++20 jewish date libray in the same style of std::chrono date. As the compilers i tried dont have std::chrono date classes the library currently uses Howard Hinnant's date library.
#include <jewish/date.h>
using namespace jewish::literals;
static_assert(date::year_month_day(5781_y / Tishrei / 1) == date::year(2020) / 9 / 19);
static_assert(5781_y / Tishrei / 1 == jewish::year_month_day(date::year(2020) / 9 / 19));
$ ./examples/jcal 5781
+-------------------------------------------------------------------------------------------+
| 5781 |
+----------------------+----------------------+----------------------+----------------------+
| Tishrei | Cheshvan | Kislev | Tevet |
+----------------------+----------------------+----------------------+----------------------+
| S M T W T F S | S M T W T F S | S M T W T F S | S M T W T F S |
+----------------------+----------------------+----------------------+----------------------+
| 01 | 01 02 03 04 05 06 | 01 02 03 04 05 | 01 02 03 04 |
| 02 03 04 05 06 07 08 | 07 08 09 10 11 12 13 | 06 07 08 09 10 11 12 | 05 06 07 08 09 10 11 |
| 09 10 11 12 13 14 15 | 14 15 16 17 18 19 20 | 13 14 15 16 17 18 19 | 12 13 14 15 16 17 18 |
| 16 17 18 19 20 21 22 | 21 22 23 24 25 26 27 | 20 21 22 23 24 25 26 | 19 20 21 22 23 24 25 |
| 23 24 25 26 27 28 29 | 28 29 | 27 28 29 | 26 27 28 29 |
| 30 | | | |
+----------------------+----------------------+----------------------+----------------------+
| Shvat | Adar | Nisan | Iyar |
+----------------------+----------------------+----------------------+----------------------+
| S M T W T F S | S M T W T F S | S M T W T F S | S M T W T F S |
+----------------------+----------------------+----------------------+----------------------+
| 01 02 03 | 01 | 01 02 03 04 05 06 07 | 01 02 03 04 05 |
| 04 05 06 07 08 09 10 | 02 03 04 05 06 07 08 | 08 09 10 11 12 13 14 | 06 07 08 09 10 11 12 |
| 11 12 13 14 15 16 17 | 09 10 11 12 13 14 15 | 15 16 17 18 19 20 21 | 13 14 15 16 17 18 19 |
| 18 19 20 21 22 23 24 | 16 17 18 19 20 21 22 | 22 23 24 25 26 27 28 | 20 21 22 23 24 25 26 |
| 25 26 27 28 29 30 | 23 24 25 26 27 28 29 | 29 30 | 27 28 29 |
| | | | |
+----------------------+----------------------+----------------------+----------------------+
| Sivan | Tammuz | Av | Elul |
+----------------------+----------------------+----------------------+----------------------+
| S M T W T F S | S M T W T F S | S M T W T F S | S M T W T F S |
+----------------------+----------------------+----------------------+----------------------+
| 01 02 03 04 | 01 02 | 01 | 01 02 03 04 05 06 |
| 05 06 07 08 09 10 11 | 03 04 05 06 07 08 09 | 02 03 04 05 06 07 08 | 07 08 09 10 11 12 13 |
| 12 13 14 15 16 17 18 | 10 11 12 13 14 15 16 | 09 10 11 12 13 14 15 | 14 15 16 17 18 19 20 |
| 19 20 21 22 23 24 25 | 17 18 19 20 21 22 23 | 16 17 18 19 20 21 22 | 21 22 23 24 25 26 27 |
| 26 27 28 29 30 | 24 25 26 27 28 29 | 23 24 25 26 27 28 29 | 28 29 |
| | | 30 | |
+----------------------+----------------------+----------------------+----------------------+
A jewish year leap year has 13 months. To handle this a few more classes were added.