I use the following format in all my project and I don't experiance any problem.
yyyyMMdd HH:mm:ss.fff
lowercase y: Year,
uppercase M: Month,
lowervase d: day
Uppercase H: Hour in 24 hours,
lowercase m: minute
lowercase s: second
lowercase f: fraction (miliseconds)
example:
select * from Users where DefinedTime between '20090101 00:00:00.000' AND '20100331 23:59:59.997'