Res=foreach(j=1:length(uniquetimestampnumber)) %dopar%
   { ANALYSIS[,length(TIMESTAMP),by=list(ID)]}

TIMESTAMP    ID
8/5/2014 17:45  28808
8/5/2014 18:00  28808
8/5/2014 18:15  69821
8/5/2014 18:30  69821
8/5/2014 18:45  69821
8/5/2014 19:00  56247
8/5/2014 19:15  56247
8/5/2014 19:30  56247
8/5/2014 19:45  56247
8/5/2014 20:00  56247
8/5/2014 20:00  28808
8/5/2014 20:15  28808
8/5/2014 20:30  28808
8/5/2014 20:45  28808
8/5/2014 21:00  69821
8/5/2014 21:15  69821

library(data.table)
setDT(df)[, .N,list(Day=as.Date(TIMESTAMP, '%d/%m/%Y'), ID)]
#          Day    ID N
#1: 2014-05-08 28808 6
#2: 2014-05-08 69821 5
#3: 2014-05-08 56247 5