The angle of the sun plays a huge role in the quality of light. Being early maximizes shooting time in all-natural light.
I've had a couple models now completely blow the Golden Hour opportunity due to being late. There are two main reasons why this is unacceptable for all-natural light shoots. The first being that the low angle of the sun provides that golden hue which is really adds rich color, contrast, and sheen to skin tones - missing this negates any morning shoot when direct exposure to the sun is part of the shoot. And in places like Mumbai, a higher sun later in the morning means higher temperatures and humidity levels. In turn, these conditions make it extremely unpleasant to work in for a model due to squinting, eye-watering, and possibly runny makeup.
These are just the basics. There are other factors to consider during morning and evening shoots, like logistics.
Mumbai, as of this writing, has the most congested traffic in the world. Generally speaking, that's going to require more time allotted for late afternoon travel to reach on time.
It's easy to forget that just because you set the time of meeting to coincide exactly with official sunrise and sunset times doesn't mean you're positioning yourself to make wise use of your time to capture the maximum amount of quality images.
After arrival, there may be more time necessary to travel on foot to the exact spot to be photographed. On beaches or other remote locales, this could mean up to ten minutes or more of walking to get to ideal spots for the shoot.
Disruptions & Surprises
Being late may be the tipping point in not getting any decent shots when you discover your favorite spot is occupied by others or if for any reason you must leave due to unforeseen circumstances. Improvising a new spot to shoot may then require a lot more time to locate if there aren't any convenient alternatives nearby.
If there is even one wardrobe change, this will take a few minutes. Being late will push the limits once more in shooting in all-natural light.
My de facto scheduling for Golden Hour shoots
Naturally, I'll vary times according to known conditions and logistics. But here is how I generally schedule Golden Hour shoots.
Mornings: Arrival half-hour prior to official sunrise. If the official sunrise for Mumbai is 7:15, arrival time wold be 6:45am. This will allow for approximately 2 hours of shooting in optimal light. It's important to be punctual because usually after a couple hours the sun will be high enough in the sky to cause models to squint too often and experience more watering eyes and discomfort - which doesn't make for good photography.
Evenings: Arrival two-and-a-half hours prior to official sunset. If the official sunset for Mumbai is 6:30pm, arrival time would be 4:00pm. It's important to note that being late would drastically reduce available time to shoot, in part because in Mumbai sunsets usually occur before the official time because the sun is often blocked before the horizon by pollution.
Note: It's a given in my book that not all the shooting will be done in optimal and official Golden Hour time. To be clear, Golden Hour is a bit of a misnomer in that it usually doesn't last an hour. Also, it's a bit of a myth that one can't shoot in direct sunlight before Golden Hour times in the evening or afterwards during morning. There are ways to make this work, including my favorite cheap and dirty trick of underexposing my subject and background by using higher shutter speeds - my maximum shutter speed is 1/8000 of a second, and I often shoot at that speed or less with my aperture all the way opened up to max. I could also stop down to f5.6 or so and really temper the light, depending on what effects I'm going for. Considering what's possible in corrections in Photoshop these days, I typically underexpose my subjects slightly anyway by up to a stop or two. Again, depending on what effects I'm wanting and lighting conditions.
Being professional means being punctual
I shouldn't even have to write these words. But here they are nevertheless. After a life-time of having this concept drilled into me by the education system and corporate America, I find it hard to believe that other adults can't be on time when they've agreed to something like a shoot. This has nothing to do with me being inflexible - I'm not scheduling my shoots for my convenience for the model's. I'm scheduling for the light. If someone's late, like more than 15 minutes late, this severely hamstrings a Golden Hour shoot. Which is why I tell any models I'm working with that if they're more than 15 minutes late I reserve the right to cancel the shoot.
If cleanliness is next to godliness, there's really no wiggle room for a similar punctuality analogy. I'll propose this addendum: If cleanliness is next to godliness, then punctuality is the god of Golden Hour.