The BigML Flatline editor is a lisp-like language that lets you add new fields to your dataset programmatically and perform a vast array of other data transformations. This code editor helps you visually test your Flatline expressions before you use them. It is available in the BigML Dashboard, from the dataset view. Click the configure option menu and select either FILTER DATASET or ADD FIELDS TO DATASET, both options lead to the Flatline editor. Then you will have to choose a JSON s-expression or a Lisp s-expression Operation, as seen below:
With the Flatline open you can edit your expression and transform your dataset as you like, e.g., date-time fields:
See the full Flatline reference here.