Unify expiry date calculation logic

[pawelperek-da]

Fixes #1943

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[ray-roestenburg-da]

Please add a test that shows that in both cases the expiration now shows correctly.

[pawelperek-da]

[fayi-da]

+1 for test. Small vitest test to complete the form, take note of the "expires" text, click the button, compare the time showed there