hw to split seq by date

tweetspersecondwindow.fs

windowBySecond (tweetsSq: seq<Tweet>) = 
    let tweet = Seq.head tweetsSq
    let currSec = (toDateTime tweet.DateStr).Second
    let tweetsForSecond = Seq.takeWhile (fun elem -> (toDateTime elem.DateStr).Second = currSec) tweetsSq |> Seq.toList
    Some(Some(tweetsForSecond.Length), tweetsSq)

           
let tweetsPerMSecond tracking =
   
    //|> Seq.map (fun t -> (toDateTime t.DateStr))
    Seq.unfold (fun state  -> windowBySecond state) (tweets tracking)