Customer group based on last order date prior to X

[StevetUK]

Is there a way to filter customers, and create a customer group (to contact via marketing email), based on their last order date being prior to X? E.g. so we can create a customer group comprised of people who ordered 3 or more months ago, to send a discount code to?

The Last Order filter doesn't seem to work. It includes people who ordered both 3 months ago or more AND yesterday!