We measure large expanses of time with milestones. You might have the time Before Graduation and the time After Graduation. If you're married, you'll have Before Marriage and After Marriage. Some of us refer to the time When My Parents Were Alive. A few might recall events Before the Accident or Illness.
This gives us an experiential basis for remembering events. It's how our species must've remembered events before inventing a device to divide time into various (and arbitrary) periodic intervals.
I've started to use milestones for shorter intervals of time, too. I buy cat food approximately once per month (for dry food) or once every three weeks (for canned wet food). So I might wonder how old a package of cheese is based on how many cans of unopened cat food are left in the cabinet. I perform other activities monthly, such as paying bills. But bill paying lacks the total change of setting that buying the cat food has, so it's of little use as a frame of reference.
Weekends are great milestones. They represent a time when I do not go to work. But they tend to blur together, so there are only two meaningful weekend intervals for me: Before Last Weekend and Since Last Weekend.
I've grown fond of an elegant (and free) Android app called Bodhi Timer, which was designed as a timer for meditation. The idea is that you set it so that it chimes nicely at the end of your meditation. But it also has a "restartable mode" enabling it to chime at intervals, too. I like to use the timer to keep track of time during my morning routine. I'll set it to chime at intervals of ten, fifteen or even twenty minutes. Shorter times are good for those mornings when I'm especially distracted. When I hear the chime, I reassess what I'm doing. If I'm getting ready for work, that's good. If I standing still listening to a radio program, well....
And another app that I keep running constantly is called Chime Time, which chimes like a grandfather clock hourly and once every half hour, too. I seem to respond much better to auditory prompts than to visual ones, so this method of marking time suits me. Chime Time is also free, but you can purchase alternate clock sounds as add-ons.
What are some milestones that you use to measure time?