PHP: calculate days since event and seconds since midnight

Question

This is a two part problem which should be trivial but date and time handling in PHP seems to be anything but and everything I've tried so far has either given incorrect results or crashed my program

I'm trying to replicate the following two SQL Server commands in PHP

1. Count the days since the start of the millennium

   select (cast (DATEDIFF(day,'2000-01-01',getdate()) as int) 
   

2. Count the number of seconds since midnight

   datediff(second,convert(date,getdate()),getdate())
   

I've tried all combinations of date_diff, getdate, strotime and more but nothing seems to give me a properly ISO formatted datetime or a workable method of calculating days and seconds elapsed.

I'm using PHP7 so should have all built-in functions up to date.

What am I missing?

edit: sample input data.

1. today's date in format '2020-11-22 16:57:10.112' a given date in format '2000-01-01 00:00:00.000'

expected output data : 7631 days

2. today's date in format '2020-11-22 16:57:10.112' previous midnight in format '2020-11-22 00:00:00.000'

expected output data : 61215 seconds

Answer 1

It's rather easy to do if you know your way around DateTime:

function daysSinceStartOfMillennium(DateTimeImmutable $date): int
{
    $millenniumStart = new DateTimeImmutable('2000-01-01');

    return $date->diff($millenniumStart)->days;
}

function secondsSinceMidnightOfDate(DateTimeImmutable $date): int
{
    $midnightToday = new DateTimeImmutable('today');
    $diff = $date->diff($midnightToday);

    return $diff->s // seconds
        + $diff->i * 60 // minutes to seconds
        + $diff->h * 60 * 60 // hours to seconds
    ;
}

You could also modify the functions to take date strings as arguments and create a DateTime object inside them.

I opted to create a descriptive variable inside the millennium function to better convey the solution. The creation of this variable can be omitted if you wish and the argument passed directly into the return statement:

return $date->diff(new DateTimeImmutable('2000-01-01'))->days;

Note that if you only need to use these function for the current date, they can be simplified to take no arguments:

function daysSinceStartOfMillennium(): int
{
    $millenniumStart = new DateTimeImmutable('2000-01-01');

    return (new DateTimeImmutable())->diff($millenniumStart)->days;
}

function secondsSinceMidnight(): int
{
    $midnightToday = new DateTimeImmutable('today');
    $diff = (new DateTimeImmutable())->diff($midnightToday);

    return $diff->s // seconds
        + $diff->i * 60 // minutes to seconds
        + $diff->h * 60 * 60 // hours to seconds
    ;
}