Date plus one day with update attribute

Hey everyone,

I want to make an API request for every single day in a period.

I have defined a start date and an end date in the Generate Flow File Procesor

the api structure is as follows

https://example.com/interfaces?session=${SessionId}&date_start=${since}&date_end=${since}

and now I want to loop with route on attribute and update attribute to request all days in this time period.

now the question how can I, the attribute of the start date always increase by one day. with the following expression in the update attribute processore I get an empty field as result

${since:toNumber():plus(86400000):format('yyyy-MM-dd')}

Result

Does anyone have an idea what adjusted we