Kraken has the datetime of the current payment period start.
Aside from that the only way to capture months data is by reading from Chat or pubsub when a sub occurs (or a resub shares their sub in chat).
If you are using Webhooks you get the timestamp for the event when it occured without it needing to be shared in chat.
It’s covered here
https://dev.twitch.tv/docs/api/webhooks-reference#topic-subscription-events
And works as a normal helix endpoint, but the documentation is missing,
But call
https://api.twitch.tv/helix/subscriptions/events
Kraken is a totally different API and I believe works in a totally (and perhaps no longer fully working) different way to Helix, the data models are different hence whats returned is different
So if you are trying to do “all the subs from today” your best bet is to collect in real time using PubSub or Chat.