a = times('00:00:00', '00:15:00', '01:45:00', '23:45:00')

1 2 8 96

as.numeric(cut(strptime(a, format = "%T"), breaks = "15 mins"))
#[1]  1  2  8 96

as.numeric(cut(as.POSIXct(a, format = "%T"), breaks = "15 mins"))

a <- c('00:00:00', '00:15:00', '01:45:00', '23:45:00')

cumsum(c(1, diff(strptime(a, format = "%T")) / 15))
#[1]  1  2  8 96