The best explanation of that Day of the Dove “three years” point that I’ve ever come up with is using the Klingon dating system from the novels (which I call the Kenson-Bennett Klingon calendar) and a little imagination.
Under the Klingon calendar, which I explain here (https://startreklitverse.com/klingon-calendar.php) Errand of Mercy happens at the tail end of Year of Kahless 892. Day of the Dove is midway through Year of Kahless 894. Now suppose the Klingons, or at least some of them, count that as three years. 892, 893, and 894.
This idea occurred to me as an equivalent to how the ancient Jews counted days, as per Easter Sunday being the third day from Good Friday.
Under the Klingon calendar, which I explain here (https://startreklitverse.com/klingon-calendar.php) Errand of Mercy happens at the tail end of Year of Kahless 892. Day of the Dove is midway through Year of Kahless 894. Now suppose the Klingons, or at least some of them, count that as three years. 892, 893, and 894.
This idea occurred to me as an equivalent to how the ancient Jews counted days, as per Easter Sunday being the third day from Good Friday.