Shaka and Samvat Era

[hzhao at sas.upenn.edu]

I heard that the Shaka Era began in 78 CE, and that one converts from 
Shaka Era to CE by adding 78 or 79,  depending on the month.  But this 
will make, say July 78 CE, fall in year 0 of Shaka Era.  Is this the 
case?  Also when did the Samvat era begin and how should one convert 
between it and CE and BCE?

Also,  Caitanya Mahaprabhu is said to have appeared on the Purnima night of 
phalguna masa of 1407 Shaka Era,  and I heard it is Feb 18, 1486.  But 
the end of phalguna masa doesn't usually seem to occur that early.  
Another account is that it was the 23rd of phalguna masa.  Does anyone 
have any authoritative source on this?  

Thanks,
Chuck Zhao