Adjective:
граждански, цивилен, слепоочен, преждевременно, ненавременен, незрял, неразвит, суров, неоформен.
Adverb:
преждевременно, нетипични за сезона, рано, предварително.
...I realized his UNTIMELY demise provided me with the opportunity to replace him...
He was knighted almost immediately after his master's UNTIMELY death.
I married her Aunt Lysa shortly before my beloved's UNTIMELY death.