Checking Time Values

Hi andersona1983,

Feel like I am a little late to the party, but given your data I think the following will provide the 'Prev' columns you need
andersona1983 - Ammended to include overlap column and removed formatting as the comparison needs to compare the actual dates and not just the time strings. You can format the columns in another column if required

Prev End Time = 
    var client = [Client Name]
    var index = [Index]
return
    CALCULATE(MIN('Table'[Appointment End Datetime]), FILTER(ALL('Table'), 'Table'[Client Name] = client && 'Table'[Index] = index -1))

Prev Start Time = 
    var client = [Client Name]
    var index = [Index]
return
    (CALCULATE(MIN('Table'[Appointment Start Datetime]), FILTER(ALL('Table'), 'Table'[Client Name] = client && 'Table'[Index] = index -1)))

Overlap = 
if( 'Table'[Appointment Start Datetime]< 'Table'[Prev End Time] || 'Table'[Appointment Start Datetime] = 'Table'[Prev End Time], "Overlap" , "")

Hope this Helps,

Richard

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