For example, if you have a Mask="f", this allows you to enter long-date and short-time. The allowed value should look like this: "Sunday, December 30, 2018 12:00 PM". In other words the seconds and milliseconds are not available to enter and they should be 0. However, if you previously enter a value that contains seconds or milliseconds different than 0, they are cached in the internal LastKnowValidValue property and used when you enter a new value. So, the new value will re-use the seconds and milliseconds from the previous one.