Example: Change date format
To modify date format go to Admin → System → Localization → Language Definitions and filter grid by date_format key
Once filtered change all date_format entries with preferred format for example:
for short date change d/m/Y to m/d/Y - where d - The day of the month (from 01 to 31)/ m - A numeric representation of a month (from 01 to 12)/ Y - A four digit representation of a year. By the way, if you change Y to lower y this will change year to two digit representation of a year (29/08/2011 to 29/08/11). If you change d to upper D this will turn day into a textual representation of a day (three letters) like 29/08/2011 to Mon/08/11. If you want to display the day of the month without leading zeros (1 to 31) change d (or D) to j
for long date change l dS F Y to ....... where l is (lowercase 'L') - A full textual representation of a day, dS is the date with a suffix, like 23rd or 15th, F - full textual representation of a month (January through December) and Y - four digit representation of a year. Also you can try to experiment with other specific formats at your own risk.