Icon for fixed vs variable / floating intervals

(Let me know if this should be posted elsewhere)

What kind of icon would you use to show a fixed vs. variable/floating schedule? I'm working on something for preventative maintenance specifically, but it could be used for scheduling as well.

• Fixed: happens on a schedule. For example, maintenance every 1000 miles is scheduled for 1000 miles, 2000 miles, 3000 miles, etc. Even if you weren't able to get the maintenance until the odometer was at 1800 miles, you've got another maintenance at 2000 miles.
• Variable/floating: happens in intervals, based on what has actually happened. For example, if maintenance is scheduled every 1000 miles: you get maintenance done at 1800 miles, and your next maintenance is for 2800 miles. If you missed that maintenance and got it done at 2900 miles, your next maintenance is for 3900 miles.

What kind of icon idea would be best for this? Would an icon even work for a concept as abstract as this?

Ideas:

• Timeline with equal ticks or space between ticks
• Hourglass / clock (variable)
• Calendar (fixed)