如何从/向日期减去/添加天数? [英] How to subtract/add days from/to a date?

问题描述

我正在尝试构建文件夹来存储数据拉取.我想用拉取数据的日期标记文件夹.

I'm trying to build folders to store data pulls. I want to label the folders with the day of that data in the pull.

例如.我从 mysql 中提取 5 天前的数据，我想将文件夹命名为 5 天前的日期.

Ex. I pull 5 days ago data from mysql i want to name the folder the date from 5 days ago.

MySQL 可以轻松处理日期算法.我不确定 R 到底是怎么做的.我应该在 POSIXct 中减去适当的秒数，然后转换为 POSIXlt 来命名文件夹 MM_DD_YYYY 吗?

MySQL can easily handle date arithmetic. I'm not sure exactly how R does it. Should i just subtract the appropriate number of seconds in POSIXct and then convert to POSIXlt to name the folder MM_DD_YYYY?

或者有更好的方法吗?

推荐答案

只需减去一个数字:

> as.Date("2009-10-01")
[1] "2009-10-01"
> as.Date("2009-10-01")-5
[1] "2009-09-26"

由于 Date 类只有几天，您可以对其进行基本的算术运算.

Since the Date class only has days, you can just do basic arithmetic on it.

如果您出于某种原因想使用 POSIXlt，那么您可以使用它的插槽:

If you want to use POSIXlt for some reason, then you can use it's slots:

> a <- as.POSIXlt("2009-10-04")
> names(unclass(as.POSIXlt("2009-10-04")))
[1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  "wday"  "yday"  "isdst"
> a$mday <- a$mday - 6
> a
[1] "2009-09-28 EDT"

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