how to set casio tk 2300 date/time

I forgot to mention the till was in use throughout the day . Then I re programmed the date/ time on the till [evening, 07 /12/2009] back to the actual current date/time to evening 06/12/2009 . then I let the till run its business day . Then when I ran end of day report it showed a really random value like £2700 sales in the till .

I presume this is because it added the ghost (07/12/2009) futures transactions with todays ( 06/12/2009) transaction.