The Collaborative International Dictionary of English v.0.48:

Whilom \Whi"lom\ (hw[imac]"l[u^]m), adv. [AS. hw[imac]lum,
   properly, at times, dative pl. of hw[imac]l; akin to G.
   weiland formerly, OHG. hw[imac]l[=o]m, See While, n.]
   Formerly; once; of old; erewhile; at times. [Obs. or Poetic]
   --Spenser.
   [1913 Webster]

         Whilom, as olde stories tellen us,
         There was a duke that highte Theseus.    --Chaucer.
   [1913 Webster]