IF formula (or switch) with ranges

Notice how the formula works:

Time Passed = 
SWITCH(
    TRUE(),
    DATEDIFF(MAX('Random Date'[Date]),TODAY(),DAY)> 90, "Older than 90",
    DATEDIFF(MAX('Random Date'[Date]),TODAY(),DAY)> 60, "Older than 60",
    DATEDIFF(MAX('Random Date'[Date]),TODAY(),DAY)> 30, "Older than 30",
    FALSE()
)

If it is > 90 days, then do Older than 90. 

If it is > 60 days, then do Older than 60. 

Notice I didn't have to tell it >60 and <90. I don't need that, because if it is >90, it never gets to condition #2.

So you could modify it this way for your other conditions:

Time Passed = 
SWITCH(
    TRUE(),
    MAX(SomeTable[Probability]) = 80, "30 Days",
    DATEDIFF(MAX('Random Date'[Date]),TODAY(),DAY)> 90, "Older than 90",
    DATEDIFF(MAX('Random Date'[Date]),TODAY(),DAY)> 60, "Older than 60",
    DATEDIFF(MAX('Random Date'[Date]),TODAY(),DAY)> 30, "Older than 30",
    DATEDIFF(MAX('Random Date'[Date]),TODAY(),DAY)> 15, "30 Days",
    FALSE()
)

SWITCH just keeps going through steps until it finds a it. If Probability = 80, it returns "30 days" and skips all other comparisons.