Modules & VBA :: Find Last Day Of Month From A Date Field

I'd like to further automate our invoicing system and need a field which has the last day of the month an item was completed.

Currently we have a field in the table called [Date Done]. I'm planning on adding a further field [Tax_Point].

I'd like the field to select the [date done] value and enter the final day of that month, unless, the final date of that month is in the future. in which case it would need the current date.

We create invoices at sporadic times of the month, and in the next month for the previous month; hence the need for a system date check.