You can try ParseExact method to convert a custom string to a DateTime, then use ToString method to convert it to your desired string format.
var result = DateTime.ParseExact("10:30AM", "hh:mmtt", CultureInfo.InvariantCulture)
.ToString("hh:mm:ss tt");
//result : "10:30:00 AM"
In the DateTime formatting you may remember these notes:
- hh: hour part
- mm: minute part
- ss: second part
- tt: represent AM/PM part
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solved Convert string Time format to Time format [closed]