How do you get time shifts in Dino run

Time shifts are only found on challenge mode, to get them, you must collect eggs:

30 eggs gives you the first time shift,
75 eggs gives you the second,
150 (im not sure if it's 150, haven't played it in a while. Most likely someone who edit this will will give you a more accurate answer.) gives you the third,
250 (I think) gives you the fourth.
And so on...