Feature #15742
Updated by kaishuu0123 (Koki Oyatsu) about 5 years ago
## Reproduce process
```
irb(main):002:0> require 'date'
=> true
irb(main):003:0> Date.new(2019, 5, 1).jisx0301
=> "H31.05.01"
```
* ruby version: trunk
## Result of reproduce process
```
irb(main):003:0> Date.new(2019, 5, 1).jisx0301
=> "H31.05.01"
```
## Expected result and the reason why you expect
```
irb(main):002:0> require 'date'
=> true
irb(main):003:0> Date.new(2019, 5, 1).jisx0301
=> "R01.05.01"
```
### reason
* new era (Reiwa?) starts from 2019/05/01
* refs: https://github.com/ruby/ruby/blob/trunk/ext/date/date_core.c#L7049
* I can't be sure that initial letter is "R".