Eomonth last date

Try this:

New End Time = 
// It determines whether the maximum start time within the same period matches the current row's start time.
// If they match, the end time is set to the end of the current month at 11:59:59 PM.
// Otherwise, the end time is set to the next earliest start time within the same period.

VAR MaxTicketStartTime =
    // Find the maximum start time within the same period as the current row.
    MAXX (
        FILTER ( 'Table', 'Table'[Period] = EARLIER ( 'Table'[Period] ) ),
        [Start DateTime]
    )

VAR NextStart =   
    // Find the next start time that is greater than the current row's start time within the same period.
    MINX (
        FILTER ( 
            'Table', 
            'Table'[Period] = EARLIER ( 'Table'[Period] ) 
            && 'Table'[Start DateTime] > EARLIER('Table'[Start DateTime]) 
        ),
        [Start DateTime]
    )

VAR LastTicket =
    // Calculate the end of the month for the current row's start time, setting it to 11:59:59 PM.
    EOMONTH ( 'Table'[Start DateTime], 0 ) + TIME ( 23, 59, 59 )

RETURN
    // Compare the maximum start time to the current row's start time.
    // If they are equal, return the calculated end-of-month time; otherwise, return the next start time.
    IF ( 
        MaxTicketStartTime = 'Table'[Start DateTime], 
        LastTicket, 
        NextStart
    )