Convert a date range to an interval description

Question

A requirement in a recent project was to report when a resource would be fully consumed. As well as the exhaustion calendar date I was asked to show the remaining time in English-like format, something like "1 year, 3 months to go."

The built-in DATEDIFF function

Returns the count ... of the specified datepart boundaries crossed between the specified startdate and enddate.

If used as-is this could produce misleading or confusing results. For example, using an interval of YEAR would show 1999-12-31 (YYYY-MM-DD) and 2000-01-01 to be one year apart whereas common sense would say these dates are separated by only 1 day. Conversely using an interval of DAY 1999-12-31 and 2010-12-31 are separated by 4,018 days while most people would see "11 years" as a better description.

Starting from the number of days and calculating months and years from there would be prone to leap year and size-of-month errors.

I got to wondering how this could be implemented in the various SQL dialects? Example output includes:

create table TestData(
    FromDate date not null,
    ToDate date not null,
    ExpectedResult varchar(100) not null); -- exact formatting is unimportant

insert TestData (FromDate, ToDate, ExpectedResult)
values ('1999-12-31', '1999-12-31', '0 days'),
       ('1999-12-31', '2000-01-01', '1 day'),
       ('2000-01-01', '2000-02-01', '1 month'),
       ('2000-02-01', '2000-03-01', '1 month'),              -- month length not important
       ('2000-01-28', '2000-02-29', '1 month, 1 day'),       -- leap years to be accounted for
       ('2000-01-01', '2000-12-31', '11 months, 30 days'),
       ('2000-02-28', '2000-03-01', '2 days'),
       ('2001-02-28', '2001-03-01', '1 day'),                -- not a leap year
       ('2000-01-01', '2001-01-01', '1 year'),
       ('2000-01-01', '2011-01-01', '11 years'),
       ('9999-12-30', '9999-12-31', '1 day'),                -- catch overflow in date calculations
       ('1900-01-01', '9999-12-31', '8099 years 11 months 30 days');  -- min(date) to max(date)

I happen to be using SQL Server 2008R2 but I am interested to learn how other dialects would handle this.

Answer 1

The following solution is for SQL Server. The approach is similar to Serg's in that the query uses only the DATEADD and DATEDIFF functions. It does not, however, account for negative intervals (FromDate > ToDate), and it derives years and months from the total month difference:

WITH
  MonthDiff AS
  (
    SELECT
      t.FromDate,
      t.ToDate,
      t.ExpectedResult,
      Months = x.Months - CASE WHEN DAY(t.FromDate) > DAY(t.ToDate) THEN 1 ELSE 0 END
    FROM
      dbo.TestData AS t
      CROSS APPLY (SELECT DATEDIFF(MONTH, t.FromDate, t.ToDate)) AS x (Months)
  )
SELECT
  t.FromDate,
  t.ToDate,
  t.ExpectedResult,
  Result = ISNULL(NULLIF(ISNULL(x.Years  + CASE x.Years  WHEN '1' THEN ' year '  ELSE ' years '  END, '')
                       + ISNULL(x.Months + CASE x.Months WHEN '1' THEN ' month ' ELSE ' months ' END, '')
                       + ISNULL(x.Days   + CASE x.Days   WHEN '1' THEN ' day '   ELSE ' days '   END, ''), ''), '0 days')
FROM
  MonthDiff AS t
  CROSS APPLY
  (
    SELECT
      CAST(NULLIF(t.Months / 12, 0) AS varchar(10)),
      CAST(NULLIF(t.Months % 12, 0) AS varchar(10)),
      CAST(NULLIF(DATEDIFF(DAY, DATEADD(MONTH, t.Months, t.FromDate), t.ToDate), 0) AS varchar(10))
  ) AS x (Years, Months, Days)
;

Output:

FromDate    ToDate      ExpectedResult                 Result
----------  ----------  -----------------------------  -----------------------------
1999-12-31  1999-12-31  0 days                         0 days
1999-12-31  2000-01-01  1 day                          1 day 
2000-01-01  2000-02-01  1 month                        1 month 
2000-02-01  2000-03-01  1 month                        1 month 
2000-01-28  2000-02-29  1 month, 1 day                 1 month 1 day 
2000-01-01  2000-12-31  11 months, 30 days             11 months 30 days 
2000-02-28  2000-03-01  2 days                         2 days 
2001-02-28  2001-03-01  1 day                          1 day 
2000-01-01  2001-01-01  1 year                         1 year 
2000-01-01  2011-01-01  11 years                       11 years 
9999-12-30  9999-12-31  1 day                          1 day 
1900-01-01  9999-12-31  8099 years 11 months 30 days   8099 years 11 months 30 days 

Answer 2

This answer shows an implementation using a SQL Server (2005+) CLR function.

-- Enable CLR (if necessary)
EXECUTE sys.sp_configure 
    @configname = 'clr enabled',
    @configvalue = 1;

RECONFIGURE;

Assembly and function

CREATE ASSEMBLY DBA
AUTHORIZATION dbo
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WITH PERMISSION_SET = SAFE;
GO
CREATE FUNCTION dbo.IntervalDescription
(
    @From date, 
    @To date
)
RETURNS nvarchar(100)
AS EXTERNAL NAME 
    DBA.UserDefinedFunctions.IntervalDescription;

Usage

SELECT 
    TD.FromDate,
    TD.ToDate,
    TD.ExpectedResult, 
    IntervalDescription = dbo.IntervalDescription(TD.FromDate, TD.ToDate) 
FROM dbo.TestData AS TD;

Result

Source

_{I am not a C# programmer!}

using Microsoft.SqlServer.Server;
using System;
using System.Text;

public partial class UserDefinedFunctions
{
    [SqlFunction
        (
        DataAccess = DataAccessKind.None,
        SystemDataAccess = SystemDataAccessKind.None,
        IsDeterministic = true,
        IsPrecise = true,
        Name = "IntervalDescription"
        )
    ]
    [return: SqlFacet(IsFixedLength = false, IsNullable = false, MaxSize = 100)]
    public static string IntervalDescription(DateTime From, DateTime To)
    {
        var workDate = From;
        int years = To.Year - From.Year;
        int months = 0;
        int days = 0;

        if (years != 0)
        {
            if (From.Month > To.Month || (From.Month == To.Month && From.Day > To.Day))
            {
                years--;
            }
            workDate = workDate.AddYears(years);
        }

        while (workDate < To && (workDate.Year != DateTime.MaxValue.Year || workDate.Month != DateTime.MaxValue.Month))
        {
            if (workDate.AddMonths(1) <= To)
            {
                months++;
                workDate = workDate.AddMonths(1);
            }
            else
            {
                break;
            }
        }

        while (workDate < To)
        {
            days++;
            workDate = workDate.AddDays(1);
        }

        StringBuilder sb = new StringBuilder(100);

        if (years > 0)
        {
            sb.Append(years);
            sb.Append(years == 1 ? " year" : " years");
            sb.Append((months > 0 || days > 0) ? ", " : string.Empty);
        }

        if (months > 0)
        {
            sb.Append(months);
            sb.Append(months == 1 ? " month" : " months");
            sb.Append(days > 0 ? ", " : string.Empty);
        }

        if (days > 0 || (years == 0 && months == 0))
        {
            sb.Append(days);
            sb.Append(days == 1 ? " day" : " days");
        }

        return
            sb.ToString();

    }
}

Answer 3

My version, implemented in SQL Server 2008R2 SP2.

CREATE FUNCTION dbo.ReadableInterval(
    @FromDate AS date,
    @ToDate AS date
)
RETURNS TABLE AS RETURN 
(
with YearStep as
(
    select
        max(n1.Number) as YearNumber
    from dbo.Numbers as n1
    where n1.Number <= DATEDIFF(YEAR, @FromDate, @ToDate)  -- see comment (A)
    and DATEADD(YEAR, n1.Number, @FromDate) <= @ToDate     -- see comment (B)
)
, MonthStep as
(
    select
        max(n2.Number) as MonthNumber
    from dbo.Numbers as n2
    cross apply YearStep as y1
    where n2.Number <= DATEDIFF(MONTH, DATEADD(YEAR, y1.YearNumber, @FromDate), @ToDate)
    and DATEADD(MONTH, n2.Number, DATEADD(YEAR, y1.YearNumber, @FromDate)) <= @ToDate
)
, DayStep as
(
    select
        DATEDIFF(day, DATEADD(MONTH, m1.MonthNumber, DATEADD(YEAR, y2.YearNumber, @FromDate)), @ToDate) as DayNumber
    from MonthStep as m1
    cross apply YearStep as y2
)
select
    y.YearNumber,
    m.MonthNumber,
    d.DayNumber
from YearStep as y
cross apply MonthStep as m
cross apply DayStep as d
)

With the given test data the results are

select
    td.FromDate,
    td.ToDate,
    td.ExpectedResult,
    ri.YearNumber as Years,
    ri.MonthNumber as Months,
    ri.DayNumber as [Days]
from dbo.TestData as td
cross apply dbo.ReadableInterval(td.FromDate, td.ToDate) as ri;

FromDate   ToDate     ExpectedResult               Years Months Days
---------- ---------- ---------------------------- ----- ------ ----
1999-12-31 1999-12-31 0 days                           0      0    0
1999-12-31 2000-01-01 1 day                            0      0    1
2000-01-01 2000-02-01 1 month                          0      1    0
2000-02-01 2000-03-01 1 month                          0      1    0
2000-01-28 2000-02-29 1 month, 1 day                   0      1    1
2000-01-01 2000-12-31 11 months, 30 days               0     11   30
2000-02-28 2000-03-01 2 days                           0      0    2
2001-02-28 2001-03-01 1 day                            0      0    1
2000-01-01 2001-01-01 1 year                           1      0    0
2000-01-01 2011-01-01 11 years                        11      0    0
9999-12-30 9999-12-31 1 day                            0      0    1
1900-01-01 9999-12-31 8099 years 11 months 30 days  8099     11   30

Explanation

My general approach is to step forward from the earlier date, first in years, then months, then in days. At each level of granularity the objective is to get as close to the end date without going over it, then continue at the next lower level.

I use a numbers table to facilitate the close-to-but-not-over calculation. From this table and DATEADD I can find the largest number of years/ months/ days that precede ToDate - comment (B) in the code.

Since I was looking for the MAX number and my Numbers table is clustered on it, the optimizer was performing a descending scan, feeding values to DATEADD. This was causing date overflow errors as Numbers contains over 100,000 rows. DATEADD(YEAR, 100000, @FromDate) is greater than 9999-12-31 and an error is raised. Predicate (A) gives an upper limit on the Number value from which the backward scan starts, avoiding the date overflow. Consequently the query plan traverses very few rows for even very large date ranges.

This approach is used for finding years and months, except the starting point for months is brought forward by however many years I found in the first CTE. DAYS is my lowest level of granularity so a simple DATEDIFF is sufficient.

This could be extended to finer granularity, returning the interval in hours, minutes and seconds if required.

Answer 4

PostgreSQL supports the age function out-of-the-box:

select
  FromDate,
  ToDate,
  ExpectedResult,
  age(ToDate, FromDate)
from TestData;

This gives the desired result, give-or-take some additional time values.

FromDate      ToDate        ExpectedResult                  age
----------    ----------    ----------------------------    --------------------------
1999-12-31    1999-12-31    0 days                          00:00:00
1999-12-31    2000-01-01    1 day                           1 day
2000-01-01    2000-02-01    1 month                         1 mon
2000-02-01    2000-03-01    1 month                         1 mon
2000-01-28    2000-02-29    1 month, 1 day                  1 mon 1 day
2000-01-01    2000-12-31    11 months, 30 days              11 mons 30 days
2000-02-28    2000-03-01    2 days                          2 days
2001-02-28    2001-03-01    1 day                           1 day
2000-01-01    2001-01-01    1 year                          1 year
2000-01-01    2011-01-01    11 years                        11 years
9999-12-30    9999-12-31    1 day                           1 day
1900-01-01    9999-12-31    8099 years 11 months 30 days    8099 years 11 mons 30 days

Answer 5

Version with no number table or tally required. Gives the same result on Michael Green's test data. They differ on data where @FromDate > @ToDate. ReadableInterval2 returns negative values contrary to nulls.

CREATE FUNCTION dbo.ReadableInterval2(
    @FromDate AS date,
    @ToDate AS date
)
RETURNS TABLE AS RETURN 
(with checkData as (
    select 
       fromDate = case when @FromDate > @ToDate then @ToDate else @FromDate end,
       toDate = case when @FromDate <= @ToDate then @ToDate else @FromDate end,
       k = case when @FromDate > @ToDate then -1 else 1 end
), MonthStep as (
    select k, FromDate, ToDate,
        YearNumber = x.months / 12,
        MonthNumber = x.months % 12
    from checkdata
    cross apply(
        select months = DATEDIFF(MONTH, FromDate, ToDate)
            - case when DAY(FromDate) > DAY(ToDate) then 1 else 0 end
        ) x
)
select YearNumber = k*YearNumber, 
      MonthNumber = k*MonthNumber,
      DayNumber = k*DATEDIFF(day, DATEADD(MONTH, MonthNumber, DATEADD(YEAR, YearNumber, FromDate)), ToDate) 
    from MonthStep 
)