Repeating Days

Hi there,

I have a dataset with half-hour logs of data throughout the day, for 365 days a year. However, I need all those rows of data (each separated by 30 mins), to be grouped as a single day. For example all the logs for day 1 (48 of them) will be grouped under a single value of day 1. So far, I have highlighted all days 1s, and then do the 'merge cells' function.

I have 365 days to do this for and think there must be a more efficient way of doing things. I have attached a screenshot of my spreadsheet - The boxed up area in red is where I am trying to group the days together. In that red box, I have grouped day 11 and about to group day 12.

Any help would be much appreciated!

Thank you!