BOLT #12: better definitions of relative_timestamp.

It makes sense to have invoices expire before end of payment window if
they're in different currencies, so let them use the default paywindow too.
And add the reader requirements to match.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

12-offer-encoding.md

@@ -776,14 +776,13 @@ A writer of an invoice:
   - if the invoice corresponds to an offer with `recurrence`:
     - MUST set `recurrence_basetime` to the start of period #0 as calculated
       by [Period Calculation](#offer-period-calculation).
-    - MUST set `relative_expiry` `seconds_from_timestamp` to the number of
-      seconds after `timestamp` that payment for this period will no longer be
-      accepted.
+    - if it sets `relative_expiry`:
+	  - MUST NOT set `relative_expiry` `seconds_from_timestamp` more than the number of seconds after `timestamp` that payment for this period will be accepted.
   - otherwise:
     - MUST not set `recurrence_basetime`.
-    - if the expiry for accepting payment is not 7200 seconds after `timestamp`:
-      - MUST set `relative_expiry` `seconds_from_timestamp` to the number of
-        seconds after `timestamp` that payment should not be attempted.
+  - if the expiry for accepting payment is not 7200 seconds after `timestamp`:
+    - MUST set `relative_expiry` `seconds_from_timestamp` to the number of
+      seconds after `timestamp` that payment of this invoice should not be attempted.
   - if the `min_final_cltv_expiry` for the last HTLC in the route is not 18:
     - MUST set `min_final_cltv_expiry`.
   - if it accepts onchain payments:
@@ -849,6 +848,10 @@ A reader of an invoice:
   - MUST reject the invoice if `description` is not present.
   - MUST reject the invoice if `timestamp` is not present.
   - MUST reject the invoice if `payment_hash` is not present.
+  - if `relative_expiry` is present:
+    - MUST reject the invoice if the current time since 1970-01-01 UTC is greater than `timestamp` plus `seconds_from_timestamp`.
+  - otherwise:
+    - MUST reject the invoice if the current time since 1970-01-01 UTC is greater than `timestamp` plus 7200.
   - if `blinded_path` is present:
     - MUST reject the invoice if `blinded_payinfo` is not present.
     - MUST reject the invoice if `blinded_payinfo` does not contain exactly as many `payinfo` as total `onionmsg_path` in `blinded_path`.
@@ -915,6 +918,13 @@ Note that the recipient of the invoice can determine the expected
 amount from either the offer it received, or the invoice_request it
 sent, so often already has authorization for the expected amount.
 
+It's natural to set the `relative_expiry` of an invoice for a
+recurring offer to the end of the payment window for the period, but
+if that is a long time and the offer was in another currency, it's
+common to cap this at some maximum duration.  For example, omitting it
+implies the default of 7200 seconds, which is generally a sufficient
+time for payment.
+
 # Invoice Errors
 
 Informative errors can be returned in an onion message `invoice_error`