Never saw crystals expire before. If you have them in your inventory you could wait a year to open them if you like. The timer is there for how long the event is and the chance to earn the shards.
You should OBTAIN or CONVERT the Romance shards into actual Crystals before the timer expires. You don't need to actually Open them right away (but they probably have to be “converted” into crystals In order to remain in your crystals screen after timer ends).
Just like special hero crystals, if you bought any while they were available, they remain there even though you can not buy any more later on.
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Just like special hero crystals, if you bought any while they were available, they remain there even though you can not buy any more later on.