Automatically Converting to GMT

GMT is an acronym for Greenwich Meridian Time, which is a reference time for the world; it is the time in Greenwich, England, and is sometimes referred to as "Zulu time." (Zulu is the phonetic name for zero, and the zero refers to the longitude of Greenwich, England.)

You may have a need to convert a local time to GMT in your worksheet. If you always know that the time will be entered in local time, this can be done quite easily with a formula. For instance, assume that you are entering the local time in cell B7, and that you are in the Pacific time zone. In this time zone, you are either seven or eight hours behind GMT, depending on if daylight savings time is in effect. The following formula will adjust the time entered in B7 by either seven or eight hours, depending on whether the date associated with the time is within the period of daylight savings time.

=IF(AND(B7>=DATEVALUE("3/8/2009 02:00"),B19<=
DATEVALUE("11/01/2009 02:00")),B7+7/24,B7+8/24)

Remember that whenever you enter a time into a cell, Excel automatically attaches a date to it. Thus, if you enter a time of 10:15 into a cell, and the day you make the entry is January 17, then Excel automatically converts the entry in the cell to 01/17/2009 10:15:00. This is done even though you may only be displaying the time in the cell—in Excel, every date has a time associated with it, and every time has a date associated with it.

Because of this entry behavior, Excel would use the formula just shown to do the proper adjustment based on the default date when you enter a time (today's date) or a date you may explicitly enter.

The only drawback to this formulaic approach is that you must remember to change the daylight savings time boundary dates from year to year. (The ones in the formula are for 2009.) You could change the formula so that you actually stored the boundary dates in cells, such as E1 and E2, as follows:

=IF(AND(B7>=$E$1,B19<=$E$2),B7+7/24,B7+8/24)

While the formula is shorter, it still has a problem with the rather static determination of when daylight savings time begins and ends—you must remember to update that information manually. In addition, if you move to a different time zone, you must remember to modify the values by which the date and time are adjusted.

A really handy way around these drawbacks is to create a user-defined function that accesses the Windows interface and determines what the system settings are in your computer. Your system keeps track of daylight savings time automatically, as well as which time zone you are in. Accessing this information through a user-defined function means you will never need to worry about those items in your worksheet. You can use the following macro to do just that:

Option Explicit

Public Declare Function SystemTimeToFileTime Lib _
  "kernel32" (lpSystemTime As SYSTEMTIME, _
  lpFileTime As FILETIME) As Long

Public Declare Function LocalFileTimeToFileTime Lib _
  "kernel32" (lpLocalFileTime As FILETIME, _
  lpFileTime As FILETIME) As Long

Public Declare Function FileTimeToSystemTime Lib _
  "kernel32" (lpFileTime As FILETIME, lpSystemTime _
  As SYSTEMTIME) As Long

Public Type FILETIME
    dwLowDateTime As Long
    dwHighDateTime As Long
End Type

Public Type SYSTEMTIME
    wYear As Integer
    wMonth As Integer
    wDayOfWeek As Integer
    wDay As Integer
    wHour As Integer
    wMinute As Integer
    wSecond As Integer
    wMilliseconds As Integer
End Type

Public Function LocalTimeToUTC(dteTime As Date) As Date
    Dim dteLocalFileTime As FILETIME
    Dim dteFileTime As FILETIME
    Dim dteLocalSystemTime As SYSTEMTIME
    Dim dteSystemTime As SYSTEMTIME

    dteLocalSystemTime.wYear = CInt(Year(dteTime))
    dteLocalSystemTime.wMonth = CInt(Month(dteTime))
    dteLocalSystemTime.wDay = CInt(Day(dteTime))
    dteLocalSystemTime.wHour = CInt(Hour(dteTime))
    dteLocalSystemTime.wMinute = CInt(Minute(dteTime))
    dteLocalSystemTime.wSecond = CInt(Second(dteTime))

    Call SystemTimeToFileTime(dteLocalSystemTime, _
      dteLocalFileTime)
    Call LocalFileTimeToFileTime(dteLocalFileTime, _
      dteFileTime)
    Call FileTimeToSystemTime(dteFileTime, dteSystemTime)

    LocalTimeToUTC = CDate(dteSystemTime.wMonth & "/" & _
      dteSystemTime.wDay & "/" & _
      dteSystemTime.wYear & " " & _
      dteSystemTime.wHour & ":" & _
      dteSystemTime.wMinute & ":" & _
      dteSystemTime.wSecond)
End Function

This may look imposing, as is often the case when working with system calls, but it works wonderfully. There are three system routines referenced (SystemTimeToFileTime, LocalFileTimeToFileTime, and FileTimeToSystemTime). By setting up the calls and using them in order, the date and time are automatically adjusted to GMT. To use the function, in your worksheet you would enter this to convert the time in cell B7:

=localtimetoutc(B7)

Format the cell as date/time, and the output is exactly what you wanted.