Never write your own date parsing library
zachleat.com
Never write your own date parsing library
1–10 of 333 posts
Re: Never write your own date parsing library
#2I ran into date heck recently in a medical setting for storing birthdates. Eventually I settled on the idea that a birthdate isn’t a physical time, it’s just a string. We can force the user to enter it in the format 02/18/1993 leading zeroes and all, and operations on it other than string equality are invalid. We’ll see if this survives contact with the enemy but it’s already going better than storing and reasoning about it as a point or interval in time and people’s birthdays changing when they move timezones.
Re: Never write your own date parsing library
#3I ran into date heck recently in a medical setting for storing birthdates. Eventually I settled on the idea that a birthdate isn’t a physical time, it’s just a string. We can force the user to enter it in the format 02/18/1993 leading zeroes and all, and operations on it other than string equality are invalid. We’ll see if this survives contact with the enemy but it’s already going better than storing and reasoning a…
> people’s birthdays changing when they move timezones
That's because the developers use datetimes (aka timestamps) to store a single date. Just pick an arbitrary epoch date (such as January 1, 1900 as used by Excel, or my favorite January 1, 1600 since 1600 is a multiple of 400 making leap year calculations even simpler) and store the number of days elapsed since then. The rules involving leap years are much much simpler than rules involving timezones and timezone databases. The translation from/to this representation to a broken-down y/m/d takes only ~50 lines of code anyways.
Of course if you don't need to do arithmetic on dates, just store three numbers, year, month, and day.
Re: Never write your own date parsing library
#4I mean. Yes. Don't write your own date parsing library. Unless you want to go nuts.
https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b...
Re: Never write your own date parsing library
#5I ran into date heck recently in a medical setting for storing birthdates. Eventually I settled on the idea that a birthdate isn’t a physical time, it’s just a string. We can force the user to enter it in the format 02/18/1993 leading zeroes and all, and operations on it other than string equality are invalid. We’ll see if this survives contact with the enemy but it’s already going better than storing and reasoning a…
Even better, just make the user input dates using a calendar date picker widget instead of a text field. This gives you full control of the input.
Re: Never write your own date parsing library
#6I ran into date heck recently in a medical setting for storing birthdates. Eventually I settled on the idea that a birthdate isn’t a physical time, it’s just a string. We can force the user to enter it in the format 02/18/1993 leading zeroes and all, and operations on it other than string equality are invalid. We’ll see if this survives contact with the enemy but it’s already going better than storing and reasoning a…
> in the format 02/18/1993
Is this DD/MM/YYYY or MM/DD/YYYY? I can tell from the 18 that it's the latter, but that convention isn't universal. I'd recommend YYYY/MM/DD as a less ambiguous format, but I don't have a perfect answer.
Re: Never write your own date parsing library
#7Multiple times in my career I've had a good laugh when a non-technical manager says something along the lines of "it's just the date, how hard can it be?"
Re: Never write your own date parsing library
#8I ran into date heck recently in a medical setting for storing birthdates. Eventually I settled on the idea that a birthdate isn’t a physical time, it’s just a string. We can force the user to enter it in the format 02/18/1993 leading zeroes and all, and operations on it other than string equality are invalid. We’ll see if this survives contact with the enemy but it’s already going better than storing and reasoning a…
> in the format 02/18/1993 Is this DD/MM/YYYY or MM/DD/YYYY? I can tell from the 18 that it's the latter, but that convention isn't universal. I'd recommend YYYY/MM/DD as a less ambiguous format, but I don't have a perfect answer.
Pretty sure they force the user into that format so that shouldn't be an issue
Re: Never write your own date parsing library
#9I ran into date heck recently in a medical setting for storing birthdates. Eventually I settled on the idea that a birthdate isn’t a physical time, it’s just a string. We can force the user to enter it in the format 02/18/1993 leading zeroes and all, and operations on it other than string equality are invalid. We’ll see if this survives contact with the enemy but it’s already going better than storing and reasoning a…
> people’s birthdays changing when they move timezones That's because the developers use datetimes (aka timestamps) to store a single date. Just pick an arbitrary epoch date (such as January 1, 1900 as used by Excel, or my favorite January 1, 1600 since 1600 is a multiple of 400 making leap year calculations even simpler) and store the number of days elapsed since then. The rules involving leap years are much much si…
In my humble opinion, this is not good advice unless you demonstrably need it for query performance or something. It is very easy for the logic layer to accidentally mess that up, either in reading or, worse, in writing back.
In this case, I'd suggest storing what you mean (the user wasn't born 9,487 days after Jan 1 1970. They were born Dec 23, 1995.)
Storing the literal units (and ONLY the relevant units), as the parent has, is robust and logically+semantically correct (they could add a translation layer for UX so the user doesn't have to be particular, but that's beside the point). Whether you use a string or a struct or some date-only type is moot, as long as you're literally storing the year, month, and day, and only those three things. You can ephemerally convert it to your platform's date type if you need to.
Re: Never write your own date parsing library
#10I ran into date heck recently in a medical setting for storing birthdates. Eventually I settled on the idea that a birthdate isn’t a physical time, it’s just a string. We can force the user to enter it in the format 02/18/1993 leading zeroes and all, and operations on it other than string equality are invalid. We’ll see if this survives contact with the enemy but it’s already going better than storing and reasoning a…
> people’s birthdays changing when they move timezones That's because the developers use datetimes (aka timestamps) to store a single date. Just pick an arbitrary epoch date (such as January 1, 1900 as used by Excel, or my favorite January 1, 1600 since 1600 is a multiple of 400 making leap year calculations even simpler) and store the number of days elapsed since then. The rules involving leap years are much much si…
No, don't do that. Use a date datatype (not date/time). You aren't the first person to ever need to handle dates without times/timezones in a computer program. Use what your database/language/libraries already have to support that.