For passing times in JSON to/from a web API, why would I choose to use an ISO8601 string instead of simply the UTC epoch value? For example, both of these are the same:
Epoch = 1511324473
iso8601 = 2017-11-22T04:21:13Z
The epoch value is obviously shorter in length, which is always good for mobile data usage, and it's pretty simple to convert between epoch values and the language's local Date type variable.
I'm just not seeing the benefit to using an ISO string value.
Both are unambiguous and easy to parse in programs. The benefit of epoch like you have mentioned is that it is smaller and will be faster to process in your program. The downside is it means nothing to humans.
iso8901 dates are easy to read on their own and don't require the user to translate a number in to a recognizable date. The size increase in iso8601 is unnoticeable when compared to much much larger things like images.
Personally I would pick ease of reading over speed for an API as it will cut down on debugging time while inspecting values sent and received. In another situation such as passing times around internally you may wish to choose the speed of an integer over text so it depends which you think will be more useful.
Unix/Epoch Time
+ Compact
+ Easy to do arithmetic actions without any libraries, i.e. var tomorrow=now()+60*60*24
- Not human-readable
- Cannot represent dates before 1 January 1970
- Cannot represent dates after 19 January 2038 (if using Int32)
- Timezone and offset are "external" info, there is ambiguity if the value is UTC or any other offset.
- Officially the spec supports only seconds.
- When someone changes the value to milliseconds for better resolution, there is an ambiguity if the value is seconds or milliseconds.
- Older than ISO 8601 format
- Represents seconds since 1970 (as opposed to instant in time)
- Precision of seconds
ISO 8601 Time
+ Human readable
+ Represents instant in time, as opposed to seconds since 1970
+ Newer then Unix time format
+ Specifies representation of date, time, date-time, duration and interval!
+ Supports an offset representation
+ Precision of nanoseconds
- Less compact
- For any arithmetic actions, reach library is required (like java.time.OffsetDatetime)
Related
Time Formats
A point in time is often represented as Unix Time, or as a human-readable ISO 8601 date in UTC time string.
For example:
Unix Time
Seconds since Epoch, or Unix timestamp, in seconds or milliseconds:
1529325705
1529325705000
ISO 8601 Date
2018-06-18T15:41:45+00:00
My question
Is there a one-to-one and onto relationship between the two? In other words, is there a point in time with a single representation in one format, and more than one, or zero, representations in the other?
Yes, it is possible to find such a date. From the wiki article on Unix time:
Every day is treated as if it contains exactly 86400 seconds,[2] so leap seconds are not applied to seconds since the Epoch.
That means that the leap seconds themselves cannot be represented in Unix time.
For example, the latest leap second occurred at the end of 2016, so 2016-12-31T23:59:60+00:00 is a valid ISO 8601 time stamp. However, the Unix time stamp for the second before, at 23:59:59, is represented as 1483228799 and the second after, 00:00:00 (on January 1 2017) is 1483228800, so there is no Unix timestamp that represents the leap second.
In practice, this is probably not a problem for you; there has only been 27 leap seconds since they were introduced in 1972.
It might be worthwhile to mention that most software implementations of ISO 8601 does not take leap seconds into account either, but will do something else if asked to parse "2016-12-31T23:59:60+00:00". The System.DateTime class in .NET throws an exception, while it's also conceivable that a library would return 2017-01-01 00:00:00.
No. there is a nice correspondence between the two, but the relationship is 1 to many, and strictly speaking there may not even exist a precise Unix millisecond for a given ISO date-time string. Some issues are:
There are some freedoms in the ISO 8601 format, so the same Unix millisecond may be written in several ways even when we require that the time be in UTC (the offset is zero).
Seconds and fraction of seconds are optional, and there may be a varying number of decimals on the seconds. So a milliseconds value of 1 529 381 160 000, for example, could be written as for example
2018-06-19T04:06:00.000000000Z
2018-06-19T04:06:00.00Z
2018-06-19T04:06:00Z
2018-06-19T04:06Z
The offset of 0 would normally be written as Z, but may also be written as you do in the question, +00:00. I think the forms +00 and +0000 are OK too (forms with a minus are not).
Since there may be more than three decimals on the seconds in ISO 8601, no exact Unix millisecond may match. So you will have to accept truncation (or rounding) to convert to Unix time. Of course the error will be still greater if you convert to Unix seconds rather than milliseconds.
As Thomas Lycken noted, leap seconds can be represented in ISO 8601, but not in Unix time.
In other words, is there a point in time with a single representation in one format, and more than one, or zero, representations in the other?
No. The UTC time depends on your geographic location, ie. your latitude and longitude. However, the UNIX timestamp is a way to track time as a running total of seconds. This count starts at the Unix Epoch on January 1st, 1970 at UTC.
From Unix TimeStamp,
It should also be pointed out that this point in time technically does not change no
matter where you are located on the globe.
It occurred to me that I'm not aware of a mechanism to store dates before 1970 jan. 1 as Unix timestamps. Since that date is the Unix "epoch" this isn't much of a surprise.
But - even though it's not designed for that - I still wish to store dates in the far past in Unix format.I need this for reasons.
So my question is: how would one go about making unix-timestamps contain "invalid" but still working dates? Would storing a negative amount of seconds work? Can we even store negative amounts of seconds in a unix-timestamp? I mean isn't it unsigned?
Also if I'm correct then I could only store dates as far back as 1901. dec. 13 20:45:52 could this be extended any further back in history by any means?
Unix Time is usually a 32-bit number of whole seconds from the first moment of 1970 in UTC, the epoch being 1 January 1970 00:00:00 UTC. That means a range of about 136 years with about half on either side of the epoch. Negative numbers are earlier, zero is the epoch, and positive are later. For a signed 32-bit integer, the values range from 1901-12-13 to 2038-01-19 03:14:07 UTC.
This is not written in stone. Well, it is written, but in a bunch of different stones. Older ones say 32-bit, newer ones 64-bit. Some specifications says that the meaning is "implementation-defined". Some Unix systems use an unsigned int to extend only into the future past the epoch, but usual practice has been a signed number. Some use a float rather than an integer. For details, see Wikipedia article on Unix Time, and this Question.
So, basically, your Question makes no sense. You have to know the context of your programming language (standard C, other C, Java, etc.), environment (POSIX-compliant), particular software library, or database store, or application.
Avoid Count-From-Epoch
Add to this lack of specificity the fact that a couple dozen other epochs have been used by various software systems, some extremely popular and common. Examples include January 1, 1601 for NTFS file system & COBOL, January 1, 1980 for various FAT file systems, January 1, 2001 for Apple Cocoa, and January 0, 1900 for Excel & Lotus 1-2-3 spreadsheets.
Further add the fact that different granularities of count have been used. Besides whole seconds, some systems use milliseconds, microseconds, or nanoseconds.
I recommend against tracking date-time as a count-from-epoch. Instead use specific data types where available in your programming language or database.
ISO 8601
When data types are not available, or when exchanging data, follow the ISO 8601 standard which defines sensible string formats for various kinds of date-time values.
Date
2015-07-29
A date-time with an offset from UTC (Z is zero/Zulu for UTC) (note padding zero on offset)
2015-07-29T14:59:08Z
2001-02-13T12:34:56.123+05:30
Week (with or without day of week)
2015-W31
2015-W31-3
Ordinal date (day-of-year)
2015-210
Interval
"2007-03-01T13:00:00Z/2008-05-11T15:30:00Z"
Duration (format of PnYnMnDTnHnMnS)
P3Y6M4DT12H30M5S = "period of three years, six months, four days, twelve hours, thirty minutes, and five seconds"
Search StackOverflow.com for many more Questions and Answers on these topics.
I'm reverse engineering a query from a kibana board and it has timestamp values in it like '1408884022624'.
In reading over the elastic search date mapping docs I don't see anything in there regarding what (appears to be) some sort of millisecond or tick format. Could someone tell me what the number above represents in my query? (I'm pretty sure we're not using a custom date format.)
It's the number of milliseconds since the beginning of Unix Epoch time, 00:00:00 UTC January 1 1970. Sometimes referred to as Java Epoch time. Technically it's not Unix Epoch time as that's tracked as the number of seconds since the above date, but many tools/converters handle both seconds and milliseconds.
Care should be taken, though, as it's quite easy to accidentally get the time in one format (let's say seconds) and pass it to a function or method expecting it in the other.
http://en.wikipedia.org/wiki/Unix_time
http://www.javaworld.com/article/2074293/core-java/groovy--java--and-the-unix-epoch-time.html
I am planning on starting a project that will need to record timestamps of incoming transactions. I appreciate that Unix Time is an integer value and I can use this type of functionality to my advantage. However, Unix Time only measures in seconds. As a minimal requirement I need to record transaction times at the millisecond level.
I know that there are ways that I could get around this issue, but I was wondering if there was another standardized way of representing time data that also represented milliseconds (or, some factor of sub-milliseconds) in the time value that is fully expressed as an integer value since epoch.
Does such a time format exist? FYI, so long as the date data-type is standardized, I don't care what system this is native in. I can code my own implementation, however, I would like to use an existing date/time format, rather than create my own.
One place where such a standard is used is ECMAScript / Javascript. Javascript date objects use milliseconds since January 1, 1970, midnight UTC for their numerical integer representation. This is detailed here.
You can test this using your browser's console:
var d = new Date();
console.log(d.getTime()); // yields integer milliseconds since epoch
So yes, there is prior art for such a use.
date +%s
outputs timestamp in seconds
date +%s%N
returns timestamp in nanoseconds
To get milliseconds divide the nanoseconds by 1 000 000
UNIX time is not appropriate for time stamping transactions because it does some weird stuff, inserting leap seconds on occasion, thus making it so that you won't be able to add and subtract time stamps reliably, nor sort transactions by timestamp.
A more appropriate standard for timestamps is TAI https://www.nist.gov/pml/time-and-frequency-division/nist-time-frequently-asked-questions-faq#tai . TAI is stored in the same way as UNIX time as a number of seconds and or microseconds and or nanoseconds since the UNIX epoch, however, it is the true number, no leap seconds are added or removed. This means that you can actually add and subtract TAI timestamps to get elapsed time and TAI timestamps are always sortable. Unfortunately, support for TAI timestamps is somewhat limited. For example, linux added support for TAI timestamps only recently in version 3.10 python added this support only in version 3.9/time.html?highlight=time#module-time
Is there any sort of advantage (performance, indexes, size, etc) to storing dates in MongoDB as an ISODate() vs. storing as a regular UNIX timestamp?
The amount of overhead of a ISODate compared to a time_t is trivial compared to the advantages of the former.
An ISO 8601 format date is human readable, it can be used to express dates prior to January 1, 1970, and most importantly, it isn't prey to the Y2038 problem.
This last bit can't be stressed enough. In 1960, it seemed ludicrous that wasting an octet or two on a century number could yield any benefit as the turn of the century was impossibly far off. We know how wrong that turned out to be. The year 2038 will be here sooner than you expect, and time_t are already insufficient for representing – for example – the schedule of payments on a 30-year contract.
MongoDB's built-in Date type is very similar to a unix timestamp stored in time_t. The only difference is that Dates are a 64bit field storing miliseconds since Jan 1 1970, rather than a 32bit fields storing seconds since the same epoch. The only down side is that for current releases it treats the count as unsigned so it can't handle dates before 1970 correctly. This will be fixed in MongoDB 2.0 scheduled for release in about a month.
A possible point of confusion is the name "ISODate". It is just a helper function in the shell to wrap around javascript's horrible Date constructor. If you call either "ISODate()" or "new Date()" you will get back the exact same Date object, we just changed how it prints. You are still free to use normal ISO Date stings or time_t ints without using our constructors, but you won't get nice Date objects back in your language of choice.