Disallow negative reminder time periods, fix time util to not recognise non-0-9 digits.

**The Issue**
Negative time periods were parsed with negative signs being ignored. TimeUtil class failed to reject digits that didn't fit into the 0-9 category. 

**Justification for this PR:**
Because insufficient user feedback given when using time periods in the past.

**PR Breakdown**
Switches to a simpler comparison to check for 0-9 characters in TimeUtil class. Checks for use of '-' character in time string.