filter() question

Hi,

I have a small test sample with lab reports (PAP smears) from a number
of different providers.  These have Collection Dates and the relevant 
columns glimpse() something like this:

$ Provider       <chr> "Dr C", "Dr D", "Dr C", "Dr D"
$ CollectionDate <chr> "2016-11-03", "2016-11-02", "2016-11-03", "2016-11-03"


I am looking to find (filter) the reports which were collected in the
time period common to all providers?

Something like 

	 the largest First Common CollectionDate 
and 
	 the smallest Last Common CollectionDate

How would I do that? 

I can of course do this "manually", ie collect all Providers and their 
first and last Collection dates and then find the Common First and Last 
one, but wonder if there is an elegant way of doing this :-)-O



greetings, el

If you want to email me, replace nospam with el

Dr. Eberhard W. Lisse   \         /       Obstetrician & Gynaecologist
el at lisse.NA             / *      |  Telephone: +264 81 124 6733 (cell)
PO Box 8421 Bachbrecht  \      /  If this email is signed with GPG/PGP
10007, Namibia           ;____/ Sect 20 of Act No. 4 of 2019 may apply