There is no need to count hours at all if you work full-time, defined as "at least 30 hours per week." You simply work for a minimum of one year (52 weeks).
If you work part-time (defined as fewer than 30 hours per week), you do have to document your weekly hours, so that you can calculate your time in terms of the full-time equivalent. For example, if you work 20 hours per week for 12 weeks, that's 240 hours. If you divide 240 by 30, you'll see that you have the equivalent of 8 full-time weeks. Or, you work until you have reached the full-time equivalent of one year (1560 hours). 52 weeks * 30 hours per week = 1560 hours.
If you work 20 hours one week, 35 the next, total will be 50 hours, because you can only count up to 30 hours per week (because that is the full-time equivalent).