How to show time passing? 1 day

If it's a proper controllable set you could move the light to simulate the sun moving throughout the day, if it's a live location you'd need to composite. Depending on framing and the relative positions of the shadows and talent you could even do it without green screen, you'd just need to have the tripod absolutely locked down.

Or you film something else in the office... Like the printer... Printing out pappers during different times during the day.

Transitions: Fade to Black. Open to White.
The brain associates certain colours with moods, situations, seasons and times of the day. Use them wisely. You could even alter the grading to reflect the time of day.

Simulate the sun movement with a light or directly the natural light with the camera fix with out movement. Is the easy way.

I'd film two plates, use green screen, and do a timelapse. I believe this might be how other films do it.

1) A plate of your actor doing his thing at the desk, in standard 24p, and put green screen over the areas that will be affected by the time lapse
2) I'd then film your timelapse without the actor present

OR

I'd make sure all windows are blocked off from any light coming in. The room should hopefully be very dark. Then, light the scene and move your lights via motor or dolly to show the sun moving.

Just fade....why add a style when it doesn't add any additional narrative value....:)

You could try a moving camera approach like the (forgive me) Twilight movie:
https://www.youtube.com/watch?v=g4gEFZ0TJ8o

In a novel and creative way that advances your story with the appropriate mood and style...

I'd stay away from green screen if you can. If this works for the story - try the old music video high frame rate trick - film at a high frame rate and rapidly move the outside key light up on a crane or big jib arm. You can also dim the light up (this will change color temp as well) Have the actor move (less) quickly inside doing his actions - if the timing is right, when the high frame rate is corrected to 24fps the actor will appear to be moving normally while the sun moves quickly.

Have two lights, both connected to a dimmer: one will represent moonlight, possibly positioned behind the actor (cool color), and one will represent sunlight, possibly positioned on the side of the actors face (warm color). For daylight, keep the sunlight on, and keep the moonlight light off: vice versa for nighttime. To transition, just dim the pair in opposite directions (so increase the intensity of the moonlight simultaneously as you decrease the intensity of the sunlight, then when the moonlight is at peak intensity, reverse the direction), and bam, you have simulated a day passing without crazy keying/effects. Practical, yet effective.

film the static shot then in post colorize them into segments and either hard cut or dissolve them into each other...or if thats too stylized use a bunch of actions in the background like practical lights slowly turning on, outside light going down with time-specific actions like his girlfriend brings lunch, then his friends ask him out to go clubbing after the lights go on

though this is dependent on the story and controllable locations of course

Chroma-Key.

Shoot the actor with a green background.

Replace the green with an actual timelapse of the Sunlight passing through the window.

In an ideal world.
If it's only the sun you need then put the big light source on a circular track and move it across/around the window.

If you have to go in to night then use the same lamp and a few meters of gel start with CTO and grade over to CTB gradually. The moon wouldn't normally act continuously like that but as a trick it will work.

If you want to use the sun then pictulate it like time-lapse but more beautiful. If you use the sun, what happens if it gets cloudy half way through the day?

Christina Moumouri, a great greek cinematographer. She made that "timelapse circle construction" with 3 different kind of jells on it, in front of a light. ENJOY
https://www.facebook.com/video.php?v=343621509151397&set=vb.319950274851...

If I would have only natural light to work with, I would use a big mirror to reflect the sunlight into the room. Than I would play around with the angles and movements to find the 'sunset' and 'sunrise' pattern. Your character has to turn on an internal light soure at 'night' to have some light in the room, when the mirror doesn't reflect any.

But in any case, first you should make a video of the real phenomenon ideally in the same room, and speed it up, so you know what you're aiming for.

I would heed the advice of doing it practical (in-camera). You will get better looking results than chroma keying it.

My personal favorite is a simple match cut from night to day, or day to night. When done right, it works incredibly well.

Show paper filling up with words; cross fades showing sheets of paper being moved to a pile. Have a large clock app on the computer screen - show this with the time changing, cross fade between. Have a clock in the visible background