change variable when difference between 2 rows

I have a variable matrix, one column is datetime, I want to change a variable called lastdayclose to the previous days close price.

eg: matrix has 2 columns

datetime

close price

variable lastdaysclose:

so I want to loop through the array and pickup the close of yesterday and assign it to the variable lastdaysclose each time there is a new day:

[dpb]

As usual, it would be far simpler to have a small(ish) set of your data to work with and an example of what you expect the result to be ... here, it isn't clear what to do for other than the first element of the new day and where the result is supposed to go and, for sure, which is the input vis a vis the output.