Convert a Mm-Yy String "Jan-01" into Date Format

Convert a mm-yy string Jan-01 into date format

If you want a date class, use as.Date:

x <- "Jan-01"
as.Date(paste("01-", x, sep = ""), format = "%d-%b-%y")

If you want a POSIXct class, consider using lubridate:

library(lubridate)
x <- "Jan-01"
dmy(paste("01-", x , sep =""))

If you want to just rely on base:

x <- "Jan-01"
as.POSIXct(paste("01-", x, sep = ""), format = "%d-%b-%y")

How to convert string date (Jan 25, 2021) to y-m-d date (2021-01-01) in python

As you can see in my comment, you just need to use the right matching mask to get it right.

Your date strings are in the format %b %d, %Y, so you need to use the same mask in strptime(). With that in mind, a function like this will do the job:

from datetime import datetime

def mdy_to_ymd(d):
    return datetime.strptime(d, '%b %d, %Y').strftime('%Y-%m-%d')

And here is a proof of concept:

>>> from datetime import datetime
>>> 
>>> 
>>> def mdy_to_ymd(d):
...     return datetime.strptime(d, '%b %d, %Y').strftime('%Y-%m-%d')
... 
>>> mdy_to_ymd('Jan 25, 2021')
'2021-01-25'
>>> 

Bear in mind that strptime() creates a datetime object from a string matching a date using masking characters format. Once you get the right mask and the right representation in a datetime object, you can convert it to the desired format using strftime().

For more information, check strftime() and strptime() Format Codes.

Convert date from mm/yyyy to mm/yy using either date-fns or moment

Implementation using moment.js

import Moment from "moment";

let datestring = "08/2021";
let date = Moment(datestring, "MMYYYY");
let datedot = date.format("MMMM YYYY");
alert(datedot); // Output - August 2021

Stackblitz - https://stackblitz.com/edit/typescript-kgrhff?file=index.ts

Convert string to date, format: dd.mm.yyyy

The format is case-sensitive ("%y" is ambiguous and system dependent, I believe):

as.Date(D, "%d.%m.%Y")
[1] "1948-12-06"

The help topic ?strptime has details:

 ‘%y’ Year without century (00-99).  On input, values 00 to 68 are
      prefixed by 20 and 69 to 99 by 19 - that is the behaviour
      specified by the 2004 and 2008 POSIX standards, but they do
      also say ‘it is expected that in a future version the default
      century inferred from a 2-digit year will change’.

Java string to date conversion

That's the hard way, and those java.util.Date setter methods have been deprecated since Java 1.1 (1997). Moreover, the whole java.util.Date class was de-facto deprecated (discommended) since introduction of java.time API in Java 8 (2014).

Simply format the date using DateTimeFormatter with a pattern matching the input string (the tutorial is available here).

In your specific case of "January 2, 2010" as the input string:

1. "January" is the full text month, so use the MMMM pattern for it
2. "2" is the short day-of-month, so use the d pattern for it.
3. "2010" is the 4-digit year, so use the yyyy pattern for it.

String string = "January 2, 2010";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MMMM d, yyyy", Locale.ENGLISH);
LocalDate date = LocalDate.parse(string, formatter);
System.out.println(date); // 2010-01-02

Note: if your format pattern happens to contain the time part as well, then use LocalDateTime#parse(text, formatter) instead of LocalDate#parse(text, formatter). And, if your format pattern happens to contain the time zone as well, then use ZonedDateTime#parse(text, formatter) instead.

Here's an extract of relevance from the javadoc, listing all available format patterns:

var dateString = "10/23/2015"; // Oct 23
var dateObject = new Date(dateString);
document.body.innerHTML = dateObject.toString();