Gregorovitch has been bragging about owning the wand, which drew the attention of the young Grindelwald.
We have just a few clues with which to narrow down the year of this event. We know that it had to have happened after the death of Ariana Dumbledore in 1899 but before the events of his capture by MACUSA in 1926. He is seen stealing the wand as a young man, which suggests that he was closer to 20 than 30 at the time. We'll narrow down the theft to between 1900 and 1910 and assign it a year of 1905.