How to get last date of a month using Autocomplete rules

Business Object – Element Duration Date 

Requirement - The Final close date for a colleague should be defaulted to end date of the pay period in which the termination date plus 3 months falls e.g., if termination date is 26 May 2023, then the final close date would be 31 August 2023. How to fetch last day of the month for this requirement.

Logic

Termination Date = 26 May 2023 

Final Close Date Logic = 26 May 2023 + 3 months = 26 August 2023 

Since 31st is the last day of August month the Final Close date should be defaulted to 31 August 2023.

Thanks,

Firoz.