@Rika 

Klaviyo only accepts certain data type when it comes to dates, please read this article. 

From what you said I think the problem is you only want to add month and day and Klaviyo doesn’t work with dates without year.. so you can add random year to all of your contacts and upload them to klaviyo under some profile property. Just make sure when uploading on the right side choose the option this property to be saved as Date and not list or string.

Hey @Rika,

Adding to @Bobi N.’s great suggestion, when importing date properties for profiles, Klaviyo only accepts certain formats as detailed in our Acceptable Date and Timestamp Formats for Profile and Event Properties Help Center article. I also went into greater details in another Community post which I’ve included below that may help!

As far as ensuring you aren't importing new contacts to Klaviyo, you’ll want to cross reference the list you are importing with those existing profiles of your Klaviyo account. Importing contacts into Klaviyo will not only update properties of existing profiles but also add new contacts to your Klaviyo account if they aren’t already an existing contact. 

David