As it was mentioned here https://restfulapi.net/http-methods/ (and in other places as well):
GET APIs should be idempotent, which means that making multiple
identical requests must produce same result everytime until another
API (POST or PUT) has changed the state of resource on server.
How to make this true in an API that return time for example? or that return data that is affected by time.
In other words, each time I use GET http://ip:port/get-time-now/, it is going to return a different response. However, I did not send any POST or PUT between two sequenced GET's
Does this make the previous statement wrong? Did I misunderstand something?
Idempotency is a promise to clients/intermediaries that the request can be reissued in case of network failures or the like without any further considerations and not so much that the data will never change.
If you take a POST request for example, in case of a network failure you do not know if the previous request reached the server but the response got lost midway or if the initial request didn't even reach the server at all. If you re-issue the request you might create a further resource actually, hence POST is not idempotent. PUT on the other side has the contract that it replaces the current representation with the one contained in the request. If you send the same request twice the content of the resource should be the same after any of the two PUT requests was processed. Note that the actual result can still differ as the service is free to modify the received entity to a corresponding representation. Also, between sending the data via PUT and retrieving it via GET a further client could have updated the state in between, so there is no guarantee that you will actually receive the exact representation you've sent to the service.
Saftiness is an other promise that only GET, HEAD and OPTIONS supports. It promises the invoker that it wont modify any state at all hence clients/intermediaries are safe on issuing such request without having to fear that it will modify any state. In practice this is an important promise to crawlers which blindly invoke any URLs in order to learn their content. In case of violating such promises, i.e. by deleting data while processing a GET request the only one to blame is the service implementor but not the invoker. If a crawler invokes such URLs and hence removes some data it is not the crawlers fault actually but only the service implementor.
As you have a dynamic value in your response, you might want to prevent caching of responses though as otherwise intermediaries might return an old state for your resource
