Formatting Dates and Timestamps for CSV Files
If you're going to be uploading a CSV of data into Klaviyo, these are the datetime formats Klaviyo will recognize. Note that all formats must include both a date as well as time component. If there's no time of day associated with your date, you can set it to be midnight by using the HH:MM:SS value 00:00:00.
For example, the date September 1st, 2014 at 1:34:08pm should be formatted like one of the following:
How to Upload These Properties into Klaviyo
Below is an example of how you would upload your master list with custom properties "Date Added - Old ESP," "Last Open - Old ESP," and "Last Click - Old ESP" with the correct datetime format: