Brian Dunning's FileMaker Custom Functions

isDaylightTime ( dt )

Returns 1 or 0 to indicate whether the given date falls within U.S. daylight savings time (post-2007)

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Howard Schlossberg   Howard Schlossberg - Show more from this author
FM Professional Solutions

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  Sample input:
isDaylightTime ( date( 5; 1; 2014 ) )
  Sample output:

  Function definition: (Copy & paste into FileMaker's Edit Custom Function window)

Returns 1 or 0 to indicate whether the given date falls within U.S. daylight savings time (post-2007)



Scott   Scott, Edmonton, AB
Apr 29, 2015
There is a mistake in the DSTEnd calc., it should be identical to the DSTBegin calc except for the month/day values.

Instead of simply fixing it that way, I made the following modification

dow_oset = floor( mod( 1 + ( yr * 1.25); 7) );
DSTBegin = date( 3 ; 14 ; yr ) - dow_oset;
DSTEnd = date( 11 ; 7 ; yr ) - dow_oset

An interesting detail to note is DST is a constant at 238 days. Therefore DSTEnd can be obtained thusly:

DSTEnd = DSTBegin + 238

Additionally, there is a DayOfWeek function, so the following is equivallent

dow_oset = DayOfWeek(date(3, 1, yr))

Here is the entire rewrite:

yr = Year(dt);

// DSTBegin is 2nd Sunday of March, minimum is the 8th.
FirstPossibleSunday = Date(3; 8; yr);

// Offset to 2nd Sunday of March
DSTBegin = FirstPossibleSunday + (8 - DayOfWeek(FirstPossibleSunday))

dt < DSTBegin ; 0 ;
dt >= DSTBegin + 238 ; 0 ;
Howard Schlossberg   Howard Schlossberg, FM Professional Solutions
Apr 29, 2015
Thanks for your notes, Scott. Yours is certainly easier to understand than mine, which I essentially copied from some forum I found online for some other computer language when I needed a solution for whatever I was working on at the time.

Meanwhile, I've fixed the missing floor() in my calc above, and visitors now have the option of using mine or yours.

Thanks again...

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