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.
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: