splitting time vector into days

Greetings -- I have a dataframe a with one element a vector, time, of  
POSIXct values.  What's a good way to split the data frame into  
periods of a$time, e.g. days, and apply a function, e.g. mean, to some  
other column of the dataframe, e.g. a$value?

Cheers,
Alexy