lua字符串和时间戳相互转换

1. 时间戳转成格式化字符串

直接利用函数os.date()将时间戳转化成格式化字符串.
```
local timestamp = 1561636137;
local strDate = os.date("%Y/%m/%d %H:%M:%S", timestamp)
print("strDate = ", strDate);
```
输出: strDate = 	2019/06/27 19:48:57
注意'%Y', Y是大写, 如果是小写的话则输出为:
输出: strDate = 	19/06/27 19:48:57

2. 字符串转化成时间戳

通过string.find的模式匹配, 解析子串.
```
local strDate = "2019/06/27 19:48:57"
local _, _, y, m, d, hour, min, sec = string.find(strDate, "(%d+)/(%d+)/(%d+)%s*(%d+):(%d+):(%d+)");

print(y, m, d, hour, min, sec);
```
输出: 2019	06	27	19	48	57
这里已经将'y, m, d, hour, min, sec'从strDate中分离出来了, 接下来转化为时间戳.完整代码:
```
--分离字符串
local strDate = "2019/06/27 19:48:57"
local _, _, y, m, d, _hour, _min, _sec = string.find(strDate, "(%d+)/(%d+)/(%d+)%s*(%d+):(%d+):(%d+)");

print(y);
print(m);
print(d);
print(_hour);
print(_min);
print(_sec);

--转化为时间戳
local timestamp = os.time({year=y, month = m, day = d, hour = _hour, min = _min, sec = _sec});
print("timestamp = ", timestamp);
```
输出:
2019
06
27
19
48
57
timestamp = 	1561636137