What I think is close to being the final version for this project, looks like this:

const ms = 1, sec = 1000*ms, mn = 60*sec, hr = 60*mn, day = 24*hr, mth
+ = 30.4375*day, yr = 365.25*day; 
function _DateInterval( vv ) {
    if( vv <   25*sec ) return  2.5*sec; if( vv <   50*sec ) return   
+ 5*sec;   
    if( vv <   2.5*mn ) return 12.5*sec; if( vv <  3.75*mn ) return   
+25*sec;  
    if( vv <     5*mn ) return 37.5*sec; if( vv <    10*mn ) return   
+  1*mn;   
    if( vv <    20*mn ) return     2*mn; if( vv <     1*hr ) return   
+  5*mn;   
    if( vv <   1.5*hr ) return    10*mn; if( vv <     3*hr ) return   
+ 15*mn;  
    if( vv <     6*hr ) return    30*mn; if( vv <    12*hr ) return   
+  1*hr;   
    if( vv <    18*hr ) return     2*hr; if( vv <  1.5*day ) return   
+  3*hr;   
    if( vv <  2.5*day ) return     6*hr; if( vv <    5*day ) return   
+ 12*hr;  
    if( vv <   10*day ) return    1*day; if( vv <   20*day ) return   
+ 2*day;   
    if( vv <   35*day ) return    4*day; if( vv <   70*day ) return   
+ 7*day;   
    if( vv <  140*day ) return   14*day; if( vv <     1*yr ) return   
+ 1*mth; 
    if( vv <     2*yr ) return    2*mth; if( vv <   2.5*yr ) return   
+ 3*mth; 
    if( vv <     5*yr ) return    6*mth; return 1*yr; 
}
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Suggestions -- especially re:JS -- welcomed.