Not sure if your description is spot on, "In other words, if the date exists in Table 2, the next daythat does not exist in Table 2 should be taken." If it does not exist in Table 2, use the day from table 1? Not sure why 1/1 gets a result of 1/1.
In the meantime, I imagine it will be an if with a lookup.
Let me know about the logic
If this solves your issues, please mark it as the solution, so that others can find it easily. Kudos are nice too. Nathaniel