Use the text functions left() and right() and concatenation (&).
E.g. left( ___ , 2 ) & ":" & right (___,2)
Though with time it's not that simple since you can have 345, and whatever other variation you are expecting from your source field, unless your source field uses consistent 24 hour notation e.g. 0345
Thanks, this works
It works to enter the value, but you might now have your value entered as text, not time if you set this up as an auto-enter calculation.
There are ways to do this and keep the field a field of type time that lets you do this and avoids the error message telling you that you incorrectly entered the time value.
A script using OnObjectValidate comes to mind. A set field set field step in the script can use this calcualtion.