Most families count any day with planned learning: lessons at home, a co-op class, a field trip with a purpose, a test day. Write down what happened on the days away from the table, so the record explains itself if anyone asks.
How many days do you need?
Many states that set a number ask for 180 days; some count hours instead, or offer either. Pennsylvania, for example, accepts 180 days or 900 hours at the elementary level and 990 at the secondary level. Other states ask for no set number at all. Laws change, so check your state's current homeschool law and keep what it asks for.
Keeping it easy
Mark the day as it happens, not at the end of the month.
Use letters for days away: F for a field trip, C for a co-op, T for a test.
Total each month in the last column; the year's total is then one sum away.