Financial Year calculating wrong years

Hi, 
I am using the code below to try and work out the financial year, and for the output to be YY/YY.  It appears to be doing that but upon closer inspection, the actual year output is incorrect in some instances and I can't understand why. 

For surv_apps[surv_date] I have, as an example, 31/01/2020 which should have a financial year 19/20 but the output is 20/21. I have tried to pay around with the +1 and -1 but can't get it to output the correct FY.

Can anyone tell me please what it is that I need to amend?

SWITCH (
    TRUE (),
    ISBLANK ( surv_apps[surv_date] ), BLANK (),
    surv_apps[surv_start_month_no] = 4 && MONTH ( surv_apps[surv_date] ) >= 4, 
		FORMAT ( surv_apps[surv_date], "YY" ) & "/" & FORMAT ( surv_apps[surv_date], "YY" ) * 1 + 1,
    surv_apps[surv_start_month_no] = 4,