The date used for never-expiring exbanners varies, on retail like with Mii Maker this is December 31, 2099, where year is 0x833, month is 0xC, and day is 0x1F.
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The date used for never-expiring exbanners varies, on retail like with Mii Maker this is December 31, 2099, where year is 0x833, month is 0xC, and day is 0x1F. The expiration-timestamp is only used for SpotPass exbanners, not extdata-exbanners.