os.date函数定义

• 原型：os.date ([format [, time]])
• 解释：返回一个按format格式化日期、时间的字串或表。

lua源码中os.date的注释如下：

---
--- Returns a string or a table containing date and time, formatted according
--- to the given string `format`.
---
--- If the `time` argument is present, this is the time to be formatted (see
--- the `os.time` function for a description of this value). Otherwise,
--- `date` formats the current time.
---
--- If `format` starts with '`!`', then the date is formatted in Coordinated
--- Universal Time. After this optional character, if `format` is the string
--- "`*t`", then `date` returns a table with the following fields:
---
--- **`year`** (four digits)
--- **`month`** (1–12)
--- **`day`** (1-31)
--- **`hour`** (0-23)
--- **`min`** (0-59)
--- **`sec`** (0-61), due to leap seconds
--- **`wday`** (weekday, 1–7, Sunday is 1)
--- **`yday`** (day of the year, 1–366)
--- **`isdst`** (daylight saving flag, a boolean). This last field may be absent
--- if the information is not available.
---
--- If `format` is not "`*t`", then `date` returns the date as a string,
--- formatted according to the same rules as the ISO C function `strftime`.
---
--- When called without arguments, `date` returns a reasonable date and time
--- representation that depends on the host system and on the current locale.
--- (More specifically, `os.date()` is equivalent to `os.date("%c")`.)
---
--- On non-POSIX systems, this function may be not thread safe because of its
--- reliance on C function `gmtime` and C function `localtime`.
---@overload fun():string|table
---@param format string
---@param time number
---@return string|table
function os.date(format, time) end

os.date格式符对照表

os.date ([format [, time]])

由原型可以看出可以省略第二个参数也可以省略两个参数，

只省略第二个参数函数会使用当前时间作为第二个参数，

如果两个参数都省略则按当前系统的设置返回格式化的字符串，做以下等价替换 os.date() <=> os.date("%c")。

如果format以 “!” 开头，则按格林尼治时间进行格式化。

**如果format是一个 “t” **，将返一个带year(4位)，month(1-12)， day (1--31)， hour (0-23)， min (0-59)，sec (0-61)，wday (星期几， 星期天为1)， yday (年内天数)和isdst (是否为日光节约时间true/false)的带键名的表;

**如果format不是 “t” **，os.date会将日期格式化为一个字符串，具体如下：

使用示例

我的lua版本：lua5.3

print ("os.date(\"*t\") 示例:\n")
local timetable = os.date("*t", os.time());
for i, v in pairs(timetable) do
	print(i, v);
end

print ("\n \"!\" 开头:\n")
local utimetable = os.date("!*t", os.time());
for i, v in pairs(utimetable) do
	print(i, v);
end

print ("\n其它用法:\n")
print(os.date("今天是 %c, 星期 %A"))

输出结果：

今天东八区中国广州的日期为：2018-11-1 21:13 星期四

os.date("*t") 示例:
wday    5
year    2018
day     1
sec     51
hour    21
isdst   false
month   11
yday    305
min     13

os.date("!*t") 示例:
wday    5
year    2018
day     1
sec     38
hour    13  --- 差8小时
isdst   false
month   11
yday    305
min     14

今天是 11/01/18 21:15:36, 星期 Thursday

• 注意format "!" 的用法，因为我们的时间（北京）处于东8区，所以两次的结果会差8个小时(13+8=21)，从结果中可以看出。
• 注意使用format "*t"返回的table中wday如果是1表示星期天，而使用通用格式时%w用0表示星期天。

参考：https://www.jianshu.com/p/76ac11863591