If by ksh, you mean ksh93:
$ t=093500
$ DATEMSK=/dev/stdin <<<%H%M%S printf "%(%F %T)T\n" "$t"
2018-05-17 09:35:00
Or:
$ printf '%(%F)T %.8s\n' now "${t//??/\0:}"
2018-05-17 09:35:00
Stéphane Chazelas
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