how to get a normalized daily standard deviation from erratic data

#1
Hi, I use Excel to analyze data. On one of my workbooks, I have about 30,000 rows of intraday data. Sometimes I might have 0 datapoints on a particular day, some days I might have 15 datapoints, and other days 20 datapoints.

On average, I have about, 5,500 datapoints over the period of a year.

What would be the best way to arrive at a "daily" standard deviation? Should I use the square root of 5,500 in the formula?

Thanks!