Time aligning GPS data with other sensors/systems that record in UTC. I'm combining data from multiple different sources together that I do not control. The duration of the data is long enough that an occasional leap second slips in and offsets things.
Ok, so it's not "keeping them in sync" as you stated in your first post, it's not a "leap second sneaking in" as in your second post. Now it is somehow data that is not labeled, and only becomes a problem during leap seconds. I'm sure you can see how changing your story and lack of details would make someone skeptical that the problem even exists.
If there really is a problem I'm really interested in hearing what it is. But it doesn't sound like you plan to be straight forward about explaining it.