I have a dataset that looks something like the image below.

Each part gets ordered at random intervals over the course of the year. I have just shown a sample of the Date column but imagine the Date column to have 368 days for each part (31/1/2020 to 02/09/2021). This interval is fixed. When the part gets ordered, the corresponding value in the Part and Quantity fields is populated but when not, both are Null. I would like to replace the Null values in the Part column with the appropriate Part number for that year.
Please note, that it is possible that a part gets ordered on say 20th of June 2021 and then does not get ordered at all, and the next part gets ordered on the 10th of October, 2020. In this case all the parts and Quantity fields from 21st of June to 9th of October will be Null and so, the only way to know when a parts' cycle is completed is by looking at the date values.
Appreciate your help! 🙂