Redmine should be able to understand ":30" or ":5" in the Estimated time field of issues
I think the current validation is too strict, and redmine should only look for a number and ":" to be contained in the field.
A value of ":30" should be interpreted as 30 minutes and a value of ":5" as five minutes.
There's really no need to ask the users to enter a leading zero for validation purposes.
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