If something as changed and approximately when.

Good evening everyone,

I’m in the final stages of completing my PIP review form but I’m struggling with something!
On the form it asks if something has changed and when approximately it changed. It’s this when approximately that I’m not sure of. My condition has slowly deteriorated and it was only due to an MRI that my concerns of a deterioration were discovered.
Would you advise that I put this date or explain that there has been a gradual deterioration without stipulating a time frame/date?

Thank you in advance,

Ally