Measurement of Time - Math Fun Worksheets A day has 24 hours. That is exactly what a 24 hour clock reads. In a 12 hour clock, 24 hours in a day is divided into two periods. Its AM and PM. AM stands for Ante meridiem and PM stands for Post Meridiem. AM means before noon and PM is after noon. Thus 8 AM means 8′ o clock in the morning and 6 PM means 6′ o clock in the evening.