What you're referring to is called a cusp, which is the moment one sign changes to the next. The cusp between Aquarius and Pisces happens at a precise moment, which varies from year to year. If you were born before it you would be Aquarius, and after it you would be Pisces.
The moment of a cusp is not related to calendar dates. It's not as simple as saying February 18th is always Aquarius and February 19th is always Pisces. One year the cusp may be at 10.12pm at night on the 18th, the next year it may be early in the morning on the 19th etc.
To find out for sure which sign you are we need to know the time, date, year and location you were born. If you can tell me that information I can tell you for sure which sign you are. Feel free to Contact me (also available from the menu of any page of this site)
If you don't know your birth time then things get a little more complicated, but it is often possible to figure it out "backwards" from major life events. That's a technique called birth time rectification. I won't confuse things here by going into it, other than to say it exists for that scenario.