What time will it be 1 hour from now? Add one hour to the time on your clock and leave the minutes exactly as they are. At 4:12 PM, the answer is 5:12 PM the same day.
How long until 1 PM depends only on the time on your clock right now. If it is 11:18 AM, you have 1 hour and 42 minutes to go. If it is 12:53 PM, you have 7 minutes left. And if 1 PM already slipped by today, the countdown simply rolls over to tomorrow's 1 PM instead.
Wondering what date is 20 weeks from today? Take today's date, add 140 days, and you have it. Twenty weeks always equals exactly 140 days, so the math never changes, only the starting point does.
Trying to work out how long until 12 AM? Glance at your clock now, and count forward to the moment it flips to 00:00. At 9:15 PM that gap is 2 hours 45 minutes. At 11:52 PM it has shrunk to 8 minutes. Because midnight is one fixed instant rather than a stretch of time, the number keeps dropping every single second.
What time was it 15 hours ago? Take fifteen hours off the time on your clock and leave the minutes untouched. At 9:38 PM, the answer is 6:38 AM the same day.
What time will it be 18 hours from now? Add eighteen hours to the time on your clock, and keep the minutes exactly as they are. At 6:15 AM, the answer is 12:15 AM the next day.
How long until 11 AM depends only on the clock in front of you right now. If it is 9:05 AM, there are 1 hour and 55 minutes left. If it is 10:40 AM, only 20 minutes remain. Once the clock passes 11:00 AM, the countdown resets and starts pointing to 11 AM tomorrow instead.
Planning something nine weeks out? What date is 9 weeks from today comes down to one simple move. Add 63 days to today's date. That works because 9 weeks always equals 9 Γ 7 days. No month-length guessing needed.
Trying to work out how long until 9:45 AM? Pick up your phone, glance at the clock, and count forward until you hit 9:45. At 7:20 AM, that gap is 2 hours and 25 minutes. Once the clock has already gone past 9:45, the wait simply resets and points to tomorrow's 9:45 AM.
Planning something further out? What date is 7 weeks from today comes down to one small piece of math: add 49 days to today's date. Since a week is always seven days, 7 weeks is always 49 days, no matter which month you start in. Count forward 49 days on the calendar and you land on your answer, weekday and all.