/
gets replaced by your culture's date separator which seems to be -
.
Read: The "/" Custom Format Specifier:
The "/" custom format specifier represents the date separator, which is used to differentiate years, months, and days. The appropriate localized date separator is retrieved from the DateTimeFormatInfo.DateSeparator property of the current or specified culture.
You can use CultureInfo.InvariantCulture
:
string dateAsString = DateTime.Now.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture);