=EOMONTH(DATEVALUE("2011-01-01"),3)

The function itself is correct. The issue is that the text date is being interpreted with a different date system/locale, so DATEVALUE("2011-01-01") is not being converted to the intended 2011-01-01 serial date, which then makes EOMONTH return a completely different (far future) date.

To fix it in Web Excel:

1. Enter the start date as a real date value in a cell, not as a text literal inside DATEVALUE.
   • For example, type 2011-01-01 into cell A1 and ensure it is right‑aligned (recognized as a date).
2. Use EOMONTH directly on that cell:

      =EOMONTH(A1,3)
   

3. If the locale expects a different date format, type the date in that format (for example 2011/01/01 or 01/01/2011) so that Excel recognizes it as a date.
4. If dates are stored as text and not recognized, convert them to real dates first (for example, using Text to Columns or a DATE formula) and then apply EOMONTH.

Once the start date is a valid Excel date value in the correct system, =EOMONTH(start_date,3) will return 2011‑04‑30 instead of a year in the 4000s.

References:

• How to correct a #VALUE! error
• Change the date system, format, or two-digit year interpretation
• WorksheetFunction.EoMonth(Object, Object) Method
• WorksheetFunction.EoMonth method (Excel)
• EDate and EOMonth functions
• EOMonth Function (Access custom web app)