Hello Bek Aspinall,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
According to your description, you want to be able to give others access to your calendar from a specific date onwards, without seeing the past. After thorough research and consulting, it’s been concluded that the feature you are asking about is not available currently. However, thank you for bringing this to our attention, we will surely raise you concerns to our Product Team and hopefully this can be included in the future updates.
In the meantime, we suggest you submit or vote to UserVoice Pages - Microsoft Support which is a specific support site where Microsoft collects suggestions/concerns about the products.
Hope the information above is useful for you. If you still have any concern, welcome to get back to use with the updates, we will be glad to assist you further.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Sherry | Microsoft Community Moderator