elsewhen
At some other time or times; somewhen else.
archaic
And thus for this time I will take my leave of your mastership, purposing elsewhen to trouble you with the talk of longer letters, if I may learn that your gentleness will warrant my boldness therein.
Where it comes from
From Middle English ellys whan; corresponding with else + when.
Adapted from the Wiktionary entry elsewhen (CC BY-SA 4.0).