Windows Forms

DIR . /B /O:-D /T:C

DIR [drive:][path][filename] [/A[[:]attributes]] [/B] [/C] [/D] [/L] [/N]
  [/O[[:]sortorder]] [/P] [/Q] [/S] [/T[[:]timefield]] [/W] [/X] [/4]
  [drive:][path][filename]

  /O          List by files in sorted order.
  sortorder    N  By name (alphabetic)       S  By size (smallest first)
               E  By extension (alphabetic)  D  By date/time (oldest first)
               G  Group directories first    -  Prefix to reverse order
  /T          Controls which time field displayed or used for sorting
  timefield   C  Creation
              A  Last Access
              W  Last Written

FOR /F "delims=" %%I IN ('DIR . /B /O:-D /T:C') DO COPY "%%I" .\new.txt & GOTO :END
:END
START /MAX new.txt

@echo off
REM generate next PO NNN.2011.txt where NNN is a number

SET FILENAME=PO 156.2011.txt
REM SET FILENAME=PO ABC.2011.txt
ECHO Initial filename %FILENAME%

REM get 2nd token using '.' and ' ' delimiters
for /F "tokens=2 delims=. " %%a in ("%FILENAME%") do SET /a NNN=%%a

REM what did we get
ECHO NNN %NNN%

REM Increment the value
SET /a NNN=%NNN% + 1
ECHO NNN+1 = %NNN%

SET FILENAME=PO %NNN%.2011.txt
ECHO New filename %FILENAME%
PAUSE

@echo off
REM generate next PO NNN.2011.txt where NNN is a number

SET FILENAME=PO 156.2011.txt
REM SET FILENAME=PO ABC.2011.txt
ECHO Initial filename %FILENAME%

REM get 2nd token using '.' and ' ' delimiters
REM Store token as both number and text
for /F "tokens=2 delims=. " %%a in ("%FILENAME%") do SET /a NNN=%%a & SET TEXT=%%a

REM what did we get
ECHO NNN %NNN%  TEXT %TEXT%

REM Should be the same if the token is a decimal number
IF %NNN%==%TEXT% (
  ECHO Yippee, got a number
  
  REM Increment the value
  SET /a NNN=%NNN% + 1

  ECHO NNN+1 = %NNN%
  SET FILENAME=PO %NNN%.2011.txt

  ECHO New filename %FILENAME%
) ELSE (
  ECHO Darn it, got a non numeric token
)

PAUSE