。
Next April will mark ten years that I've worked here.
Literal
Next-year [possessive-の] April [at-で] 10-year here [at-で] worked-[ことになる] (it-will-be-the-case).
~ことになる expresses cumulative duration anchored to a future point. 来年の４月で ('as of next April') is the temporal anchor. 働いた + ことになる rephrases the perfect-duration sense — 'it will have become the case that I worked ten years.' Japanese arrives at English's perfect tense by stacking past form + ことになる, rather than via a dedicated grammatical tense.