Dies könnt ihr in eure environment.rb eintragen.

meine_formate = {
    
:mein_format_1 => '%l %p, %b %d, %Y',
    
:mein_format_2 => '%l:%M %p, %B %d, %Y'

} 



ActiveSupport::CoreExtensions::Time::Conversions::DATE_FORMATS.merge!(meine_formate)

ActiveSupport::CoreExtensions::Date::Conversions::DATE_FORMATS.merge!(meine_formate)

Für euer angepasstes Format könnt ihr die zugelassenen Einstellungen, die auch für Ruby gelten, nutzen.
Ausschnitt aus der Ruby-Dokumentation:

%a - The abbreviated weekday name (``Sun'')
%A - The full weekday name (``Sunday'')
%b - The abbreviated month name (``Jan'')
%B - The full month name (``January'')
%c - The preferred local date and time representation
%d - Day of the month (01..31)
%H - Hour of the day, 24-hour clock (00..23)
%I - Hour of the day, 12-hour clock (01..12)
%j - Day of the year (001..366)
%m - Month of the year (01..12)
%M - Minute of the hour (00..59)
%p - Meridian indicator (``AM'' or ``PM'')
%S - Second of the minute (00..60)
%U - Week number of the current year,
starting with the first Sunday as the first
day of the first week (00..53)
%W - Week number of the current year,
starting with the first Monday as the first
day of the first week (00..53)
%w - Day of the week (Sunday is 0, 0..6)
%x - Preferred representation for the date alone, no time
%X - Preferred representation for the time alone, no date
%y - Year without a century (00..99)
%Y - Year with century
%Z - Time zone name
%% - Literal ``%'' character