The time travel mechanism and its rules
Time travel romance's internal credibility depends on a mechanism with consistent rules: the standing stone that activates at specific times, the object that serves as a conduit, the involuntary slipping triggered by emotional states, the deliberate machine with specific parameters. Whatever the mechanism, its rules must be established early and applied consistently — because the rules are what create the romantic tension. The mechanism that the protagonist cannot control creates helplessness and urgency; the mechanism with a limited number of uses creates a countdown; the mechanism that depends on the protagonist's emotional state makes the love story itself a factor in whether the travel is possible. The rules should make the romantic problem harder, not easier, and should not change to solve the plot when the author needs a resolution.