Converting between units of time: 600 seconds = ___ minutes, 9 weeks = ___ days, 240 minutes = ___ hours. Seconds, minutes, hours, days and weeks, in both directions.
This is not the same skill as reading a clock, and the two are easy to confuse. Our telling time worksheets ask what time a clock shows; these ask how many minutes are in four hours. A student can be fluent at one and stuck on the other.
60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and 7 days in a week. Months and years are left out because their lengths vary.
Telling time is reading hands or digits off a clock face. Converting is arithmetic on a quantity: 240 minutes is 4 hours whether or not a clock is involved. Both are on this site, on separate pages.
Because a month is not a fixed number of days and a year is not a fixed number of weeks. Every conversion here has one exact whole-number answer, and those two do not.
Third and fourth grade. It usually follows telling time to the minute, once the units themselves are familiar.