How to calculate an average of a weather variable for a month during daytime hours only?

I need help computing an average value of a weather variable for daylight hours during one month. I have daily data from an instrument for January 2019, which can be found here (ftp://ftp.archive.arm.gov/haegelea1/217376_extractedData/ascii-csv/). The instrument measures cloud base height (which is denoted as "first_cbh" in the data). The instrument takes a measurement every 16 seconds starting at midnight. I have data as daily files for January 2019 each with time & cloud base height and various other non-needed variables. The time is measured as an offset from midnight in interval of 16 seconds. I need to calculate an average of the "first_cbh” column for the entire month but only during daylight hours. I am having difficulty computing an average during only the daylight hours. Not sure how to approach the time variable? Not sure to just approximate daylight hours or use a function if there is one available that computes daily daylight time based on location. If so the location of the instrument is Lamont, Oklahoma.