duration
Represents a positive or negative span of time.
构造函数参数参数是传给函数的输入，写在函数名后的括号中。
Creates a new duration.
You can specify the duration using weeks, days, hours, minutes and seconds. You can also get a duration by subtracting two datetimes.
#duration(
days: 3,
hours: 12,
).hours()
定义定义这些函数和类型带有附属定义。要访问这种定义，请先写上函数或类型的名称，再加上定义的名称，并用句点在中间分隔。
seconds
The duration expressed in seconds.
This function returns the total duration represented in seconds as a floating-point number rather than the second component of the duration.
minutes
The duration expressed in minutes.
This function returns the total duration represented in minutes as a floating-point number rather than the second component of the duration.
hours
The duration expressed in hours.
This function returns the total duration represented in hours as a floating-point number rather than the second component of the duration.
days
The duration expressed in days.
This function returns the total duration represented in days as a floating-point number rather than the second component of the duration.
weeks
The duration expressed in weeks.
This function returns the total duration represented in weeks as a floating-point number rather than the second component of the duration.