在Java中，我们大多数情况下格式化日期都是使用simpleDateFormat,比如把一个日期格式化成：2019-12-31的形式，我们一般定义模板为：yyyy-MM-dd的形式。
我们需要注意的是，这里的字母大写和小写的意义是不一样的，关于日期模板，关于定义信息java最近这几版都有些许变动，具体变动如下，（加粗红色显示），一句话概述，大写Y和小写的Y不是一个东西！
Java 1.6定义如下：
| Letter | Date or Time Component | Presentation | Examples |
|---|---|---|---|
G |
Era designator | Text | AD |
y |
Year | Year | 1996; 96 |
M |
Month in year | Month | July; Jul; 07 |
w |
Week in year | Number | 27 |
W |
Week in month | Number | 2 |
D |
Day in year | Number | 189 |
d |
Day in month | Number | 10 |
F |
Day of week in month | Number | 2 |
E |
Day in week | Text | Tuesday; Tue |
a |
Am/pm marker | Text | PM |
H |
Hour in day (0-23) | Number | 0 |
k |
Hour in day (1-24) | Number | 24 |
K |
Hour in am/pm (0-11) | Number | 0 |
h |
Hour in am/pm (1-12) | Number | 12 |
m |
Minute in hour | Number | 30 |
s |
Second in minute | Number | 55 |
S |
Millisecond | Number | 978 |
z |
Time zone | General time zone | Pacific Standard Time; PST; GMT-08:00 |
Z |
Time zone | RFC 822 time zone |
|
Java 1.7中新增的定义
| Letter | Date or Time Component | Presentation | Examples |
|---|---|---|---|
G |
Era designator | Text | AD |
y |
Year | Year | 1996; 96 |
Y |
Week year | Year | 2009; 09 |
M |
Month in year | Month | July; Jul; 07 |
w |
Week in year | Number | 27 |
W |
Week in month | Number | 2 |
D |
Day in year | Number | 189 |
d |
Day in month | Number | 10 |
F |
Day of week in month | Number | 2 |
E |
Day name in week | Text | Tuesday; Tue |
u |
Day number of week (1 = Monday, ..., 7 = Sunday) | Number | 1 |
a |
Am/pm marker | Text | PM |
H |
Hour in day (0-23) | Number | 0 |
k |
Hour in day (1-24) | Number | 24 |
K |
Hour in am/pm (0-11) | Number | 0 |
h |
Hour in am/pm (1-12) | Number | 12 |
m |
Minute in hour | Number | 30 |
s |
Second in minute | Number | 55 |
S |
Millisecond | Number | 978 |
z |
Time zone | General time zone | Pacific Standard Time; PST; GMT-08:00 |
Z |
Time zone | RFC 822 time zone | -0800 |
X |
Time zone | ISO 8601 time zone | -08; -0800; -08:00 |
Java 1.8中新增的定义
| Letter | Date or Time Component | Presentation | Examples |
|---|---|---|---|
G |
Era designator | Text | AD |
y |
Year | Year | 1996; 96 |
Y |
Week year | Year | 2009; 09 |
M |
Month in year (context sensitive) | Month | July; Jul; 07 |
L |
Month in year (standalone form) | Month | July; Jul; 07 |
w |
Week in year | Number | 27 |
W |
Week in month | Number | 2 |
D |
Day in year | Number | 189 |
d |
Day in month | Number | 10 |
F |
Day of week in month | Number | 2 |
E |
Day name in week | Text | Tuesday; Tue |
u |
Day number of week (1 = Monday, ..., 7 = Sunday) | Number | 1 |
a |
Am/pm marker | Text | PM |
H |
Hour in day (0-23) | Number | 0 |
k |
Hour in day (1-24) | Number | 24 |
K |
Hour in am/pm (0-11) | Number | 0 |
h |
Hour in am/pm (1-12) | Number | 12 |
m |
Minute in hour | Number | 30 |
s |
Second in minute | Number | 55 |
S |
Millisecond | Number | 978 |
z |
Time zone | General time zone | Pacific Standard Time; PST; GMT-08:00 |
Z |
Time zone | RFC 822 time zone | -0800 |
X |
Time zone | ISO 8601 time zone | -08; -0800; -08:00 |