Airtable formulas only have access to data in the single record, so formula fields cannot compare data across different records. Rollup fields can make limited calculations on a field value in linked records, but they also cannot provide the calculations that you want.
If you want to eyeball it for a single customer at a time, you could try using a timeline view grouped by customer, and then look for lots of records that have overlapping times. But I suspect this is not what you want.
You could calculate this with a custom script. I think a novice/beginner script writer would struggle with writing this script, but if you have experience writing scripts or know someone who can write scripts, that's the way to go.
