Moment-tz formatting issue and incorrect time output?

help
1 
import moment from 'moment-timezone';

> var guessTimeZone = moment.tz.guess()
undefined

> guessTimeZone
'Europe/London'

Summer Time:

> moment.tz('2016-05-23 09:57:00', guessTimeZone).format('YYYY-MM-DD HH:mm:ss')
'2016-05-23 09:57:00' <- this is my data format with month proceeded by 0, but incorrect time output 

> moment.tz('2016-5-23 09:57:00', guessTimeZone).format('YYYY-MM-DD HH:mm:ss')
'2016-05-23 10:57:00' <- This is what I expect as correct time, but only after having to adjust to another month input format

Winter Time:

> moment.tz('2016-12-20 09:57:00', guessTimeZone).format('YYYY-MM-DD HH:mm:ss')
'2016-12-20 09:57:00' <- Now correct

So my question is how come I needed to remove the leading zero in front of the month data format to get the right answer as I cannot modify external data from provider, unless I parse it within my app (which is something I prefer to avoid, but may have to parse using regex etc to remove leading zeros)?

Or is this a bug within the library?