How to Present Dates in Different Formats in R
You’ve probably noticed in R that as.Date() is fairly prescriptive in its defaults: It expects the date to be formatted in the order of year, month, and day. Fortunately, R allows you flexibility in specifying the date format.
By using the format argument of as.Date(), you can convert any date format into a Date object. For example, to convert 27 July 2012 into a date, use the following:
> as.Date("27 July 2012", format="%d %B %Y")  "2012-07-27"
This rather cryptic line of code indicates that the date format consists of the day (%d), full month name (%B), and the year with century (%Y), with spaces between each element.
Here are some of the many date formatting elements that you can use to specify dates. You can access the full list by typing ?strptime in your R console.
|%Y||Year with century.|
|%y||Year without century (00–99). Values 00 to 68 are prefixed by 20, and values 69 to 99 are prefixed by 19.|
|%m||Month as decimal number (01–12).|
|%B||Full month name in the current locale. (Also matches abbreviated name on input.)|
|%b||Abbreviated month name in the current locale. (Also matches full name on input.)|
|%d||Day of the month as a decimal number (01–31). You don’t need to add the leading zero when converting text to Date, but when you format a Date as text, R adds the leading zero.|
|%A||Full weekday name in the current locale. (Also matches abbreviated name on input.)|
|%a||Abbreviated weekday name in the current locale. (Also matches full name on input.)|
|%w||Weekday as decimal number (0–6, with Sunday being 0).|
Try the formatting codes with another common date format, 27/7/2012 (that is, day, month, and year separated by a slash):
> as.Date("27/7/12", format="%d/%m/%y")  "2012-07-27"