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WPierre's shop is closed on red days. Season specific tabs also mark off Festival days.
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PPlant
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H/PHarvest then plant on the same day
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HHarvest
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AgAgriculturalist (Lv.10 Farmer perk via Tiller branch)
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SGSpeed-Gro fertilizer
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DSGDeluxe Speed-Gro fertilizer
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HSGHyper Speed-Gro fertilizer
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Growth Rate Mechanics Breakdown
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The Speed-Gro fertilizers and Agriculturist perk all make your crops grow faster by some amount.
And while the in-game descriptions are good-enough approximates, how much of a bonus is actually granted with each tier of Speed-Gro (with or without Agriculturist on top) is a little more complex than a flat percentage cut in the Grow Time.

Here's how I suspect the game handles crop growth bonuses:
1. The game first adds up all applicable growth bonuses: currently, bonuses can come from Agriculturist, Speed-Gro fertilizer, and--for Unmilled Rice and Taro Root--whether it is irrigated (planted within 3 tiles of water).
2. The game takes that total (10% for Ag, 10/25/33% for fertilizer, and about 20% for irrigation) and multiplies it by the Grow Time, then rounds UP, to get the number of days reduced.*
--- Before we get to the next step, some background information: each crop has either 4 or 5 Growth Stages. Each crop spends varying numbers of days within of its stages until it's harvestable.
3. The game reduces days from each stage, starting at Stage 1 and going in order, until the number of days calculated in step 2 has been removed. If there are more days than stages, it cycles back to Stage 1 and goes again.
There is one caveat to this:
--- Stage 1 can NEVER be reduced below 1 day, because there has to be something to represent when you first plant the seed. If Stage 1 is at 1, either naturally or reduced, it is skipped, and the game moves onto Stage 2 and continues from there.
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For example, Rhubarb has the following Growth Stages for a total of 13 growing days:
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Stage 1Stage 2Stage 3Stage 4Stage 5
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Rhubarb, base22234
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If we apply Deluxe Speed-Gro (25% bonus), we get 13 * 0.25 = 3.25, which rounds up to 4. The game then removes a day from Stage 1, then a day from Stage 2, and so on until 4 days have been removed, giving us the following Growth Stages and a total of 9 growing days:
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Rhubarb, DSG11124
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That's the basic application of the mechanic. Let's look at what happens when there's a higher number of days removed. Taking Rhubarb again, let's apply Hyper Speed-Gro (33%) while having Agriculturalist (10%) for a total 43% faster growth: 13 * 0.43 = 5.59, rounding to 6. The game starts off as before, taking a day off Stage 1 then going through the rest one at a time. Once it gets to Stage 5 though, it has 2 days left to remove so it cycles back to Stage 1. This time, however, Stage 1 is already at 1 day so it skips to Stage 2 and carries on as normal, leaving us with this Growth Stage spread:
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Rhubarb, HSG+Ag10123
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Note that in either of these cases, due to the rounding, the actual amount of days taken off is higher than the stated percentage bonus. The DSG gave about 30% off (vs. 25) and the HSG+Ag comes out to be about 46% (vs.43).

There's another quirk of the game's code that can provide extra benefit to some crops. Let's look at Green Beans, base and SG/Ag (10%):
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Green Bean, base11134
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Green Bean, SG/Ag10034
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At a 10% bonus, we'd expect Green Beans to only get a single day reduction on SG/Ag. Yet at that tier, it gets 2 days off and jumps straight to Stage 4. This is due to the way the game's programming converts decimals to binary (called floating-point arithmetic), so 0.1 isn't exactly 0.1 but rather something like 0.100000001490116119384765625. That little extra bit is enough to get us an extra day due to the aforementioned rounding at certain amounts, notably the 10-day Grow Time crops.
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However, not all quirks of the programming are beneficial. Depending on the Growth Stage spread, some plants can actually lose a discounted day at high enough bonuses. Let's go back to the Green Bean, but this time on HSG with Ag: at a 43% bonus, we should be expecting a 5-day discount off the initial 10-day Grow Time. However, in-game, it still takes 6 days to fully mature. So what happened to that last day? Let's have a look at the Growth Stages:
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Green Bean, HSG+Ag1-1023
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-1?! Yes, because there isn't any rule other than the "Skip Stage 1 If It's At 1" rule, the game will actually go into the negatives on some crops with low early Growth Stages. A negative value on a Growth Stage is still treated as 0, though, because how do you have negative time.
Compare Green Bean with Coffee Bean, for example. Both have the same Grow Time of 10 days, but because Coffee spends 2 days in Stage 2, it gets the full benefit of HSG+Ag's 5-day drop.
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Coffee Bean, base12232
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Coffee Bean, HSG+Ag10121
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That should cover the ins-and-outs of the crop growth mechanics.