A DateTime in F# is an immutable object that contains both date and time information. DateTime instances are manipulated by calling their methods. Once a DateTime has been constructed, its value can never change. Any methods/functions that appear to modify a DateTime will actually return a new DateTime.
The textual representation of dates and times is dependent on the culture. Consider a DateTime with its date set to March 28 2019 and its time set to 14:30:59. Converting this DateTime to a string when using the en-US culture (American English) returns "3/28/19 2:30:59 PM". When using the fr-BE culture (Belgian French), the same code returns a different value: "28/03/19 14:30:59".
Understanding which DateTime methods are culture-dependent is important to know. In general, any DateTime method that deals with strings (either as input or output) will be dependent on the current culture.