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# \$100 Reward! Calculating upcoming billing date

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7 - App Architect

I have three fields like this:

`| Start Date | Billing | Next Billing |`

In the `Start Date` field, there’s a past date
In the `Billing` field, there’s a dropdown list with the values “Monthly”, “Quarterly”, “Semi-annually” and “Annually”
In the `Next Billing` field, I want a formula that calculates next billing date based on the values in the two previous fields. `Next Billing` should always be a upcoming date (or today).

Examples:

If `Start Date` is “2018-01-01” and `Billing` is “Monthly”, I want `Next Billing` to say “2021-09-01”
If `Start Date` is “2020-06-18” and `Billing` is “Semi-annually”, I want `Next Billing` to say “2021-12-18”
If `Start Date` is “2016-10-24” and `Billing` is “Annually”, I want `Next Billing` to say “2021-10-24”

I have tried to do this myself but I can’t work out how to do it. Can someone please help?

Thanks!

1 Solution

Accepted Solutions
18 - Pluto

Thank you! This was an interesting formula to build.

19 Replies 19
10 - Mercury

Hi Per. I’m not sure that I get your examples. In the “Monthly” case, you want to add 3 years and 9 months to the {Start Date}? In the “Semi-annually” case, you want to add 1 year and 6 months? I’m pretty sure that’s not what you mean, so I’ll answer the question that I think you’re asking. If I misunderstood, you can still use my answer, just change the SWITCH values to something else.

I just answered a question very similar to this one today. Put this formula in your {Next Billing} field.

``````DATEADD({Start Date},
SWITCH({Billing},
'Monthly', 1,
'Quarterly', 3,
'Semi-annually', 6,
'Annually', 12),
'months'
)
``````

You can use an automation from here if you want to update {Billing} at the end/start of a cycle. Good luck.

7 - App Architect

Hi!

It’s not that I want to add a specific amount of months in each example, I want the formula to calculate the next upcoming (future) billing date. Your formula gives me dates in the past. `Next Billing` should always be sometime in the future (or today).

Thanks!

10 - Mercury

Do you mean that you want to give the formula a {Start Date} that may be years in the past, set the {Billing} to “monthly” for example, and then have the {Next Billing} be the same day of month in the next month of this year? Just an example.

7 - App Architect

Yes, that’s exactly what I want. I just want to know “when is the next time I should send a bill to this customer based on it’s start date and billing interval?”.

10 - Mercury

Hmm…let me think…give this a try maybe?

``````DATEADD(
DATETIME_PARSE(MONTH({Start Date}) & '/' & DAY({Start Date}) & '/' & YEAR(NOW()),
'I'),
SWITCH({Billing},
'Monthly', 1,
'Quarterly', 3,
'Semi-annually', 6,
'Annually', 12),
'months'
)
``````
7 - App Architect

That gives me #ERROR! :slightly_smiling_face:

10 - Mercury

Replace the ‘I’ in the DATETIME_PARSE with ‘MM/DD/YYYY’. Let me know.

7 - App Architect

That got rid of the error. Some dates are correct, but some dates are in the past (which is wrong). I’ll PayPal you \$100 if you solve this one.

10 - Mercury

Nevermind. I see the problem. I’ll think about it.

10 - Mercury

Thanks Per. I see the issue more clearly now. I’m not sure I have the skill to solve this in a formula field. Good luck!

7 - App Architect

Thanks for trying! :slightly_smiling_face:

7 - App Architect

Anyone got any ideas? I’ll PayPal \$100 to whomever solves this problem.

18 - Pluto

Is this what you want?

Would you like to provide any more test data?
By the way, this formula is 62 lines long.

7 - App Architect

That’s EXACTLY what I want! Are there any downsides of the formula being so long?

18 - Pluto

Sometimes a formula has to be long in order to include all the logic.

One down side is that the longer the formula, the longer it takes Airtable to recalculate. On the other hand, Airtable will eventually complete the recalculations. While this formula is on the longer side, it is far from reaching the limits of Airtable’s capabilities, and it still recalculates reasonably quickly.

Another down side of long formulas is that they can be difficult to maintain, as there is no built-in way of including comments in a formula. This is why it is very important to have a quality formula to start with that covers all of your potential edge cases. If there is a small logic error in the formula that you don’t discover until after the formula is in production, it can be very difficult to root out the problem. This is also why I ask if you want to test any additional dates before purchasing the formula, while the algorithm is fresh in my mind. (I do write my formulas to make them easy to read, but even so it can be difficult to retrace the logic in these formulas after the fact due to the lack of comments.)

Mostly, I list the number of lines in the formula to indicate that I do not consider this to be a trivial formula that can be solved in only a few lines.

7 - App Architect

Thanks for clarifying! The only problem I can see in your test is the record with “Quarterly” as billing interval. `Next Billing` should be 2021-09-28, as I will bill this customer every quarter starting from March 28th. I.e March 28th, June 28th, September 28th and December 28th each year. Let me know if anything is unclear.

18 - Pluto

Thanks for the catch. This is what I mean about the importance of testing cases. I got mixed up between a quarter being 3 months versus 4 months. Lol. There are 4 quarters in a year, but each quarter should be only 3 months, not 4 months long.

7 - App Architect

Lol :slightly_smiling_face: PM me with your PayPal account and I’ll send you the money.

18 - Pluto

Thank you! This was an interesting formula to build.