how to update date to interface field in bill line in subscription product

Summary:

Hi

we are trying to update date to interface field in bill line based on charge definition

Eg : if charge definition is true up then date to interface should be added with start date + 1 month

if date to interface is 1/1/2025 then it should be 1/2/2025

we had tried before insert trigger but this is not working as expected

code :

def billLines=billLines
billLines.reset()
while(billLines.hasNext())
{
def productbillLines=billLines.next()
def chargeCheck=false
def charges=charges
charges.reset()
while(charges.hasNext())
{
def chargesRow=charges.next()
if(chargesRow.ChargeDefinition=="TRUEUP_CHARGE")
{
chargeCheck=true
}
}
if(chargeCheck)
{
if(productbillLines.BillingPeriod!=1)
{
def billdate = productbillLines.DateToInterface