[Are there] "respected" coding gurus that we can link him to [to persuade him]?

Appeals to authority are not particularly convincing; better to present an argument that is valid no matter who said it.

Here's a reductio ad absurdum that should persuade or demonstrate that your cow-orker is stuck on being "right" regardless of sensibility:

What you really need is

getFooList(String fooName, Date start, Date end, Date middle, Date one_third, JulianDate start, JulianDate end, KlingonDate start, KlingonDate end) 

You never know when you'll have to internationalize for Klingon, so you better take care of it now because it will require a lot of work to retrofit and Klingons are not known for their patience.

[Are there] "respected" coding gurus that we can link him to [to persuade him]?

Appeals to authority are not particularly convincing; better to present an argument that is valid no matter who said it.

Here's a reductio ad absurdum that should persuade or demonstrate that your co-worker is stuck on being "right" regardless of sensibility:

What you really need is

getFooList(String fooName, Date start, Date end, Date middle, Date one_third, JulianDate start, JulianDate end, KlingonDate start, KlingonDate end) 

You never know when you'll have to internationalize for Klingon, so you better take care of it now because it will require a lot of work to retrofit and Klingons are not known for their patience.