Ruby provides two classes to format date string strftime and strptime.

1. String to Date object

Method parse of class Date converts string into date object with given directives. E.g.

Date.parse('2020-01-01', '%Y-%m-%d')

The provided directives can be changed according to given string format.

Date.parse('01-01-2020', '%m-%d-%Y')
=> Wed, 01 Jan 2020
Date.parse('31-01-2020', '%d-%m-%Y')
=> Fri, 31 Jan 2020
Date.parse('01/01/2020', '%m/%d/%Y')    
=> Wed, 01 Jan 2020

---

Some methods provided by Date class

1. Find end of the month

   Date.parse('2020-01-01', '%Y-%m-%d').end_of_month
   => Fri, 31 Jan 2020
   

2. Find end of the year

   Date.parse('2020-01-01', '%Y-%m-%d').end_of_year
   => Thu, 31 Dec 2020
   

3. Find day of the calender week. Returns 1 to 7, Monday is 1

   Date.parse('2020-01-01', '%Y-%m-%d').cday
   => 1
   

4. Find day of the month.

   Date.parse('2020-01-01', '%Y-%m-%d').mday
   => 1
   

5. Find week of the year.

   Date.parse('2020-12-31', '%Y-%m-%d').cweek
   => 53
   

6. Find day of the year.

   Date.parse('2020-12-31', '%Y-%m-%d').yday
   => 366
   

7. Find lilian day number.

   Date.parse('2020-12-31', '%Y-%m-%d').ld
   => 160055
   

8. Find if given day is Sunday. Similar methods are available for all days of the week.

   Date.new(2020, 12, 31).sunday?
   => false
   

9. Find date after x days from the date

   Date.new(2020, 12, 31) + 8
   => Fri, 08 Jan 2021
   

10. Find date after x months from the date

    Date.new(2020, 12, 31) >> 8
    => Tue, 31 Aug 2021
    

.

Format directives

  %Y - Year with century (can be negative, 4 digits at least)
          -0001, 0000, 1995, 2009, 14292, etc.
  %C - year / 100 (round down.  20 in 2009)
  %y - year % 100 (00..99)

  %m - Month of the year, zero-padded (01..12)
          %_m  blank-padded ( 1..12)
          %-m  no-padded (1..12)
  %B - The full month name (``January'')
          %^B  uppercased (``JANUARY'')
  %b - The abbreviated month name (``Jan'')
          %^b  uppercased (``JAN'')
  %h - Equivalent to %b

  %d - Day of the month, zero-padded (01..31)
          %-d  no-padded (1..31)
  %e - Day of the month, blank-padded ( 1..31)

  %j - Day of the year (001..366)

Time (Hour, Minute, Second, Subsecond):
  %H - Hour of the day, 24-hour clock, zero-padded (00..23)
  %k - Hour of the day, 24-hour clock, blank-padded ( 0..23)
  %I - Hour of the day, 12-hour clock, zero-padded (01..12)
  %l - Hour of the day, 12-hour clock, blank-padded ( 1..12)
  %P - Meridian indicator, lowercase (``am'' or ``pm'')
  %p - Meridian indicator, uppercase (``AM'' or ``PM'')

  %M - Minute of the hour (00..59)

  %S - Second of the minute (00..60)

  %L - Millisecond of the second (000..999)
  %N - Fractional seconds digits, default is 9 digits (nanosecond)
          %3N  millisecond (3 digits)   %15N femtosecond (15 digits)
          %6N  microsecond (6 digits)   %18N attosecond  (18 digits)
          %9N  nanosecond  (9 digits)   %21N zeptosecond (21 digits)
          %12N picosecond (12 digits)   %24N yoctosecond (24 digits)

Time zone:
  %z - Time zone as hour and minute offset from UTC (e.g. +0900)
          %:z - hour and minute offset from UTC with a colon (e.g. +09:00)
          %::z - hour, minute and second offset from UTC (e.g. +09:00:00)
          %:::z - hour, minute and second offset from UTC
                                            (e.g. +09, +09:30, +09:30:30)
  %Z - Equivalent to %:z (e.g. +09:00)

Weekday:
  %A - The full weekday name (``Sunday'')
          %^A  uppercased (``SUNDAY'')
  %a - The abbreviated name (``Sun'')
          %^a  uppercased (``SUN'')
  %u - Day of the week (Monday is 1, 1..7)
  %w - Day of the week (Sunday is 0, 0..6)

Seconds since the Unix Epoch:
  %s - Number of seconds since 1970-01-01 00:00:00 UTC.
  %Q - Number of milliseconds since 1970-01-01 00:00:00 UTC.

Literal string:
  %n - Newline character (\n)
  %t - Tab character (\t)
  %% - Literal ``%'' character

Combination:
  %c - date and time (%a %b %e %T %Y)
  %D - Date (%m/%d/%y)
  %F - The ISO 8601 date format (%Y-%m-%d)
  %v - VMS date (%e-%b-%Y)
  %x - Same as %D
  %X - Same as %T
  %r - 12-hour time (%I:%M:%S %p)
  %R - 24-hour time (%H:%M)
  %T - 24-hour time (%H:%M:%S)
  %+ - date(1) (%a %b %e %H:%M:%S %Z %Y)

…

• References

Date.parse - https://apidock.com/ruby/v2_5_5/Date/parse/class

strptime - https://apidock.com/ruby/Date/strptime/class

strftime - https://apidock.com/ruby/Time/strftime

StackOverflow - https://stackoverflow.com/a/14619560/5235107